While adjusting administrative divisions, local governments also achieve the goal of integrating resources among regions through cross-regional development planning. The question this article seeks to answer is whether this kind of regional development planning that breaks through administrative boundaries can drive cross-regional market integration, and thus affect the overall resource allocation and economic performance of the region. Taking the Regional Planning of Chengdu-Chongqing Economic Zone officially issued by the State Council in 2011 as a quasi-natural experiment, this paper uses the synthetic control methods to estimate the impact of this cross-regional development plan on the economic growth of Sichuan and Chongqing province. Combined with panel data from prefecture-level cities, as well as empirical data on fixed assets investment and fiscal revenue in Sichuan and Chongqing, and changes in the allocation of economic affairs authority between the two regions, the mechanism of action is examined to provide insights for further promoting western development and forming a new pattern. It is found that Regional Planning of Chengdu-Chongqing Economic Zone has promoted the overall economic growth of both Sichuan and Chongqing, and has a higher role in promoting Chongqing’s economic growth than Sichuan. The mechanism test shows that cross-provincial cooperation can effectively internalize the spillover effects generated by regional competition, and alleviate the negative effects of regional competition through regional collaboration and market integration. The empirical conclusions of the article can be applied to the analysis of the cross-provincial economic zone construction that the central government has been continuously promoting in the Yangtze River Delta, Beijing-Tianjin-Hebei region and other regions. To some extent, it can be inferred that China’s economic growth since the reform and opening up is both the result of internal competition among provinces and the result of the central government’s use of cross-provincial development planning to promote regional cooperation. The conclusion of the article is that the key to shaping a new development pattern in the western region is to effectively promote cooperation among provinces in the western region, form a joint force for development, and reduce competitive internal friction.