Using compulsory license as a threat in price negotiations is an important way to obtain anti-epidemic drugs. However, the results of implementing compulsory license threats vary a lot across different countries. Since the adoption of the Declaration on the TRIPS Agreement and Public Health (Doha Declaration) in 2001, there have been twenty-four cases which implemented compulsory license as a threat in anti-epidemic drug price negotiations by seventeen countries. Among those twenty-four cases, only nine successfully got a discount on the drug price. It is still not clear that under what circumstances compulsory license threat can play a positive role. With the global outbreak of short-term epidemics and pandemics, such as the "Covid-19" pneumonia pandemic, the context in which compulsory license is applied has changed significantly. On the one hand, new types of short-term epidemics have emerged, making the implementation and enforcement of compulsory license more complex. On the other hand, the number and scope of countries in need of anti-epidemic drugs have also expanded, not only developing countries face shortages of anti-epidemic drugs, developed countries also need to obtain anti-epidemic drugs in sudden epidemic outbreaks. Existing literature has been intensively focusing on whether compulsory license can help developing countries obtain drugs in long-term epidemics such as AIDS. Although the results of those studies provide valuable implications for developing countries to combat long-term epidemics, their applicability to the growing presence of sudden global epidemic is limited. To take full advantage of compulsory license under the new development of epidemics, it is necessary to extend the scope of existing literature and explore the specific role of compulsory license in different situations. In this study, we try to divide the context of implementing compulsory license into two dimensions. The first dimension is the epidemic, including short-term such as "Covid-19" and long-term epidemics such as AIDS. The second dimension is the country which is searching for anti-epidemic drugs, including developing countries and developed countries. We argue that due to the difference between short-term and long-term epidemics and the heterogeneous challenges faced by developing and developed countries, the effectiveness of compulsory license threats varies across contexts. To explore whether compulsory license will be helpful to obtain anti-epidemic drugs, we selected the two cases of drug price negotiations by the United States and Brazil to conduct the analysis. Firstly, we review the process of implementing compulsory license in the USA and Brazil. Secondly, we summarize and compare the different characteristics of short-term and long-term epidemics and the challenges which developing and developed countries come across when using compulsory license. Thirdly, by linking both the two dimensions, we find the key factors that influence the effectiveness of using compulsory license threats and discuss their effects in different contexts. By comparing the two cases of the USA and Brazil, we identify seven key factors which influence the effectiveness of using compulsory license threats: local manufacturing capability on anti-epidemic drugs, possibilities to import generic drugs, whether other countries use compulsory license, opposition from the domestic pharmaceutical industry, opposition from the State′s economic and trade departments, economic retaliation from the patent holder and their home country, and the number of times compulsory license has been used. The effects of those seven factors vary across contexts. First, in all situations, local manufacturing capability on anti-epidemic drugs and possibilities to import generic drugs are the most important factors for ensuring the effect of compulsory license threat in drug price negotiations. Second, whether other countries use compulsory plays an enhancing role in short-term epidemics; while in long-term epidemics, the more times a country uses compulsory license the less effective it is. Third, for developing countries, economic retaliation from the patent holder and their home country will weaken the effectiveness of compulsory license threat; for developed countries, opposition from the domestic pharmaceutical industry and the State′s economic and trade departments will raise the difficulty of implementing compulsory license and weaken its effectiveness. Based on the findings, we have provided specific suggestions for developing and developed countries on obtaining drugs in both short-term and long-term epidemics.