A finite element model of the body frame of a 10.5 m bolt-on aluminum alloy pure electric city bus produced by a certain company is established as the research object. Based on the static analysis results of four typical working conditions and combined with design experience, lightweight optimization design was carried out on the body frame and chassis, including material replacement, optimization of the main aluminum profile end face shape, and optimization of the chassis structure. Then, static and modal analysis was carried out on the optimized skeleton under four typical working conditions, and the reliability of the structure was verified through a reliability test of 30000 km on the actual vehicle. By comparing the data before and after optimization, the weight of the bus skeleton has been reduced from 2539 kg to 2332.5 kg, a reduction of 206.5 kg. At the same time, the various characteristics of the bus also meet the design requirements, providing reference for weight reduction and optimization design of other bus frame structures.