Abstract: Nowadays, promoting industrial upgrading and realizing green development has become an important development direction of many cities in China. Based on the patent data of the State Intellectual Property Office, taking the green development process of 108 cities in the Yangtze River Economic Belt from 2008 to 2018 as an example, this paper discusses the impact of technological relatedness and environmental policies on regional green development, and analyzes the crowding-out effect of existing regional dirty technologies. The results show that: ①Technological relatedness can significantly promote the entry of green technologies, and new green technologies are more likely to form and develop in areas with relevant technical capabilities; ②The existence of dirty technology hinders the formation and development of new green technologies, but the technology relatedness will greatly alleviate or even reverse this negative impact (i.e. technology crowding-out effect); ③Environmental policies at both spatial scales have no direct impact on entry of new green technologies, but will have an impact on technology relatedness. Specifically, the environmental policy at the urban level will strengthen the technological relatedness, thus promoting regional green development, while the environmental policy at the provincial level will weaken the technological relatedness. The conclusions of this paper can provide a useful reference for follow-up theoretical research and policy-making.