This study constructs a quasi-natural experiment of policy synergy with the help of the “Broadband China” strategy and the smart city pilots. It also explores the incentive effect of policy synergy on digital intelligence development based on the data of 281 prefectural cities from 2010 to 2021 by using double machine learning algorithms. The results show that: ①policy synergy significantly contributes to urban digital intelligence development, and shows a better enabling effect than a single policy. ②Policy synergy can also “double wheel” the development of urban digital intelligence through market incubation mechanisms of promoting non-agricultural entrepreneurship and increasing entrepreneurial vitality, and government support mechanisms of inducing government subsidies and intensifying government attention to innovation. ③The digital intelligence driving effect of policy synergy has a “first-mover advantage” in cities with an excellent business environment and a perfect legal system guarantee, and the positive effects of policy synergy are more prominent in central cities and economic zones east of HU Huanyong.