Abstract: In the face of the realistic dilemma of low efficiency in agricultural resource utilization, long cycle of agricultural technology research and development, and great uncertainty, open innovation is an important way to realize the green development of agriculture and is the key to achieving high-quality agricultural development. To reveal the mechanism of open innovation in agricultural green development, this study constructed an evaluation index system for agricultural green development based on four aspects, ecological conservation, resource conservation, environmental friendliness, and economic benefits, and evaluated the level of agricultural green development in various regions. Then, by constructing a nonlinear adjustment model, panel data from 31 provinces (excluding Hong Kong, Macao, and Taiwan) in China from 2010 to 2020 were used to verify whether open innovation can better promote the green development of agriculture. The results indicated the following: 1) The comprehensive evaluation result of China’s green agricultural development was 0.411, indicating that the overall level of green agricultural development was relatively low, and individual differences between provinces were significant and showed an expanding trend. Beijing had the highest growth rate in the level of green agricultural development, while Heilongjiang had the lowest. Fujian, Beijing, Tibet, and Beijing had the highest evaluations in terms of ecological conservation, resource conservation, environmental friendliness, and economic benefits, respectively. 2) Open innovation and agricultural green development presented a positive “U”-shaped nonlinear relationship, that was, there was a turning point in the impact of open innovation on agricultural green development. When open innovation investment was low, it was not conducive to the green development of agriculture. When the level of open innovation exceeded 15.3761, open innovation played a significant role in promoting green agricultural development. A robustness test also confirmed the nonlinear effect of open innovation on agricultural green development. In 2020, 11 provinces, including Tibet, Hainan, and Qinghai, were still in the suppression stage, while 20 provinces, including Beijing, Shanghai, and Guangdong, were in the promotion stage. 3) Environmental regulation positively regulated the nonlinear relationship between open innovation and agricultural green development, that was, at a higher level of environmental regulation, the inflection point of the positive effect of open innovation on agricultural green development moved to the left, and the impact rate was greater. 4) Open innovation had a differential impact on agricultural green development due to the different types of open innovation and dominant types, both inbound and outbound open innovation had a positive “U”-shaped nonlinear relationship with agricultural green development, but compared with outbound open innovation, inbound open innovation had a stronger driving effect on agricultural green development. At the same time, when outbound open innovation was greater than inbound open innovation, open innovation had a stronger driving effect on agricultural green development. According to the research results, it was proposed that areas with low open innovation should increase the cultivation of new agricultural business entities and improve absorption capacity, while areas with high levels of open innovation should focus on the construction of open innovation platforms and accelerate the flow of open innovation resources. Scientific formulation of environmental regulation policies, and improvement of environmental regulation intensity should be reinforced in low open innovation areas.