Abstract:     With the development of the Internet and the emergence of platform firms, "platform economy" and "platform ecosystem" frequently appear in the public. To promote the development of the platform ecosystem, platforms are committed to innovation and therefore have made remarkable achievements. For example, Alibaba′s innovative businesses in the e-commerce business, cloud computing, and digital media have facilitated transactions and social interactions among lots of platform participants. Xiaomi′s IoT developer platform has cooperated with partners to build an innovative ecosystem and provided smart products and services to 68 million households worldwide. Apple′s iOS and Google′s Android platform ecosystems have attracted numerous developers and end users. Existing relevant researches focus on the definition, characteristics, opening strategy, competition strategy, and governance strategy, but lack of overview research on the topic of platform ecosystem innovation. It is difficult to fully understand the platform strategy to promote ecosystem innovation, which brings trouble to the practice and theory. It is necessary to comprehensively review and analyze the literature on platform ecosystem innovation to promote the flourishing of the innovative practice and theory of the platform ecosystem. 
    In this paper, the measurement and statistics of literature clarify the research situation in this field. From the descriptive statistics of the number of publications and published journals in the past years, it is concluded that this topic has a broad research prospect. The results of keyword co-occurrence and emergence detection by CiteSpace 6.1.R6 in 1964 literature show that: First, the keywords with the highest co-occurrence frequency at home and abroad are "ecosystem" and "innovation" respectively. Second, the highest intensity of keywords at home and abroad are "platform enterprise" and "business ecosystem" respectively, which are frontier topics. Furthermore, it sorted out some opinions of key literature and got such meaningful conclusions: First, platform ecosystem in the digital economy refers to a collection of innovative organizations that platform firms as the core connects lots of complementors and end users with digital technologies and standards. Second, from the perspective of the value creation mechanism, the platform ecosystem can be divided into transaction-driven and innovation-driven. Third, transaction-driven value creation follows the service-dominant logic. Platform participants interact in the integration of resources and design of matching rules, which drives the value co-creation of all parties. Innovation-driven value creation follows the value-adding logic, which refers to the platform sponsors providing boundary resources, including software toolkits, application program interfaces, rules, etc., to support the innovation of complements, so that they can create diversified products or services, and realize the value co-creation with the platform. Fourth, this paper summarizes strategies of ecosystem innovation promoted by these two platforms, that is, platform ecosystem innovation is driven by openness, competition, cooperation, and governance strategies. Alibaba′s retail and Xiaomi′s IoT developer platform ecosystems exemplify these strategies.
    Future research on platform ecosystem innovation can focus on the following three directions: First, research on platform ecosystem innovation can enrich the Chinese local context. From the view of the number of publications and journals, domestic research is falling behind foreign studies, while the platform economy in China has been developing rapidly in recent years. Given the inconsistency between theory and practice, it is a valuable direction for scholars to explore the unique source of value creation and the platform-enabled innovation strategies, and form the platform ecosystem innovation theory with Chinese characteristics. Second, some exploration is made to the mechanism of the relationship among openness, competition, cooperation, and governance strategies and their influence on innovation. For example, the interrelated policies, such as how platforms implement governance policies by determining the degree of openness, affect ecosystem innovation, is very worthy of discussion. The existing literature on platform strategies only discussed the direct influence on the platform ecosystem innovation but has not explored its impact path. Solving the mystery of the black box would clarify the mechanism of each strategy′s impact on innovation. Third, from the perspective of government scientific supervision, the strategies for maintaining orderly competition among various platform organizations and promoting the high-quality innovative development of the platform market are meaningful research topics. It makes sense to find methods to scientifically regulate the platform market, which are helpful to guide the high-quality development of the platform ecosystem under the new pattern from the macro level.

Key words:  platform strategy, ecosystem innovation, transaction-driven, innovation-driven