Based on literature review and theoretical mechanisms, this article selects city-level data to explore the impact of provincial capital cities boundary expansion on the quality of regional green and low-carbon development. The following conclusions were obtained: (1) Overall, the expansion of provincial capital city boundaries can significantly improve the quality of green and low-carbon development in the later stages of policy implementation, which in turn significantly improves the benefits of urban green and lowcarbon development. The estimated coefficients of policy effects show a trend of slow increase followed by a significant increase. (2) The analysis of Hefei City as a case sample discovered, that expansion of the provincial capital city boundaries can bring about the policy effect of about 0.002-0.371 units to Hefei. From the perspective of the development trend, in the early stage of policy implementation (2011-2014), the degree of policy effect enhancement is slow, and the positive green low-carbon effect has not been fully utilized. In the middle and late stages of policy implementation (2015-2019), the green low-carbon effect shows an increasing development. The policy effect coefficient peaked in 2016, with a value of 0.371. (3) After the implementation of the “strong provincial capital” strategy, the efficient utilization of infrastructure construction, the reduction of industrial transfer, the positive externalities of industrial agglomeration, and the expansion of the city’s financial autonomy are the mechanism paths that generate positive green and low-carbon effects in the later stages of policy implementation.