The low-carbon pilot policy has played an important role when China make efforts to achieve the carbon emission reduction targets. In order to study whether the construction of low-carbon pilot policy can induce enterprises to carry out green technology innovation, this paper uses China′s low-carbon pilot policy as a quasi-natural experiment, constructs a difference-in-difference-in-differences (DDD) model that "before-after the implementation of the low-carbon pilot policy; if or not in the pilot city; if or not belongs to the high carbon emission industry". Since the third batch of low-carbon pilot cities was officially announced in 2017, the duration of the policy is not enough for us to evaluate its effect, so we take the pilot cities announced in 2010 and 2012 as the main research object. The number of enterprises authorized green patents and micro-economic characteristics of listed companies in China from 2006 to 2017 were used to evaluate the effect of the low-carbon pilot policy on enterprises′ green technology innovation, and explore whether the Porter hypothesis based on China′s low-carbon pilot policy is valid.

The results show that: (1) The implementation of the low-carbon pilot policy has significantly improved the green technology innovation level of high-carbon emission enterprises in the pilot cities. The policy has increased the proportion of green patents of high-carbon emission enterprises in the pilot area by 0.929%, and the coefficient is significant at the 5% significance level. Furthermore, the low-carbon pilot policy has a positive effect on both green invention patent and green utility model patent of high-carbon emission enterprises in the pilot area, but has a more significant effect on the green utility model patent. The conclusion of the DDD estimation confirms the validity of the Porter hypothesis based on the low-carbon pilot policy in China.

(2)In order to solve the problem of sample selection bias caused by the difference in initial conditions between the treat group and the control group, this paper introduces the propensity score matching method (PSM), and adopts the propensity score matching-DDD (PSM-DDD) method for the first time to explore the effect of the low-carbon pilot policy on green technological innovation of high carbon emission enterprises within pilot cities. The effectiveness of the matching process is proved through the matching balance test and drawing the propensity score value density function graph. The results of the PSM-DDD are basically similar to those obtained by the DDD method. At the same time, this paper proves the validity of the conclusions through parallel trend hypothesis test and a series of robustness tests.

(3)The analysis of enterprises geographic heterogeneity shows that the low-carbon pilot policy has significantly improved the green technology innovation of high carbon emission enterprises within the pilot cities in the eastern and western regions, but for those in the central region, the low-carbon pilot policy even has a negative inhibitory effect on their green technology innovation. After grouping the data by patent types, we can see that, for green invention patents, the low-carbon pilot policy can significantly promote the green invention patents of high-carbon emission enterprises in the pilot cities in the eastern and western regions, but it has no significant impact on enterprises in the central region; for green utility model patents, the low-carbon pilot policy only has a positive effect on the utility model patents of high-carbon emission enterprises in the pilot cities in the eastern region, but has no significant impact on the green utility model patents of enterprises in the central and western regions. 

(4)The analysis of enterprise ownership heterogeneity shows that the low-carbon pilot policy has significantly improved the green technological innovation of non-state-owned high-carbon emission enterprises in the pilot cities, but has no significant impact on the green technology innovation of state-owned enterprises in the pilot cities. Among them, the low-carbon pilot policy has a significant positive effect on promoting both green invention patents and utility model patents of non-state-owned high-carbon emission enterprises in the pilot cities. As for the state-owned high-carbon emission enterprises in the pilot cities, the low-carbon pilot policy only has a positive effect on their green utility model patents, but has no significant impact on green invention patents.

The results of this paper further expand the study on the Porter hypothesis and the evaluation of the low-carbon pilot policy.