Clarifying the optimal relative diversification level of regional producer services is conducive to optimizing the spatial layout of regional developing producer services under the context of the construction of a unified national market and promoting the construction of the Chinese-style modern industrial system. Taking China’s three Megalopolis of Bohai Rim, Yangtze River Delta, and Guangdong-Fujian as examples, through theoretical analysis and empirical research, this study explores the mechanism by which the relative diversification level of urban producer services affects economic growth, as well as the moderating role of infrastructure and advanced industrial structure. This study finds that there exists an optimal relative diversification level of producer services in cities that is favorable to economic growth. The proper development of relative diversification of producer services in cities following dynamic comparative advantage can foster economic growth. To leverage the positive moderating role of infrastructure and industrial structure on the relative diversification development performance of urban producer services, and thereby enhance the optimal relative diversification level of producer services, it is necessary to ensure that the stock of infrastructure capital and the level of advanced industrial structure are in good coordination with the relatively diversified development of producer services. The findings of this study can provide an important theoretical and decision-making reference for the policy practice of optimizing the spatial layout and enhancing the development performance of the urban producer services in the Megalopolis.