Dynamic and economic performance play an important role in the vehicle evaluation indexes, and also are crucial parts considered during the design progress. This paper applies a method of parameters matching of powertrain to one Pure Electric City Bus(PECB), with which a four-speed transmission dynamic system is designed. Meanwhile two powertrain models adopting motors with identical peak power but different base speeds and peak torques coupled with two-speed transmission or without transmission are introduced for comparison to analyze if a smaller motor performs better in performance. The three models are based on identical type of bus. Then according to the powertrain parameters, simulation models of three bus models are established respectively. From the results of simulation, vehicle performances of the four-speed transmission powertrain, especially economic performance, are compared with the other two. Results show that the four-speed dynamic system is the most optimal amongst three types. For testing the four-speed powertrain further and proving the superiority, it is loaded onto the real bus to collect data of performances. After analysis of data obtained, the four-speed powertrain has a favorable performance. Therefore, the powertrain form of a smaller motor coupled with a transmission which has more gears is proved more efficient in energy saving for PECB.