Turning counties into districts (TCID) is a policy of administrative division adjustment in order to alleviate the strain on land resources, promote urbanization and speed up regional economic integration. But its multidimensional impact on regional economy and its mechanism remains to be further investigated and verified. Therefore, under the help of Difference-in-Difference Method, the paper takes Guangdong province as the research area and uses the panel data of 79 counties (cities) from 2000 to 2015 to analyze and solve the above problems. The results show that: TCID significantly promotes the regional economic growth. Generally, its contribution to the regional economic growth rate was 0.48%; the policy effect performed better in the Pearl River Delta and less stably in other regions. The embodiment of policy dividend has time lag, and generally and emerges in the third or four year. In addition, TCID can get the infrastructure, planning, policies and other strong support. Meanwhile, increasing financial revenue and investment in fixed assets helps the development of secondary industries and related industries. Finally, it is effective to promote regional economic growth by raising employment rates and stimulating consumption. The policy recommendations of this article aim to, with the opportunity to reform the administrative system, organically combine the development of industry with the service industry, especially the productive service industry, speed up the optimization and upgrading of the local industrial structure so as to promote the sustained, healthy and rapid development of the regional economy.