Coping with climate change requires promoting low-carbon energy transition (LCET) in cities. However, the planning method of LCET for rapidly growing cities deserves further study because it involves dynamicity and interactions of multiple factors. This paper aims to put forward a comprehensive methodology to fill that gap. First, a theoretical framework of “energy system–sustainability–governance–operation of rapidly growing cities” is put forward to explain the general mechanism of LCET. Second, a three step method is built for LCET planning, including energy system analysis based on low emissions analysis platform modeling, operation analysis applying multilevel perspective and stakeholder theories, and governance evaluation by policy review. To verify this method, a city in Western China, Chengdu, was selected as the case study. The results show that Chengdu’s energy related CO2 emissions are expected to peak in 2025 under timely and aggressive measures. The main obstacles lie in three aspects: techno–economic insufficiencies, lack of social cognition, and problems of institution and policy. To realize this scenario, Chengdu must incorporate the target and pathway of a carbon peak as soon as possible into its policy system and enhance the coordination among governmental departments.