The performance index of the 250-350 km/h programmable EMU traction system was introduced. The formula for calculating total basic resistance of the EMUs (Electric Multiple Units) was determined by using the integrated Datong-Xi'an high-speed railway test data and the Chinese standard EMUs interconnection resistance test results. And the formula of the total resistance of EMUs was adjusted by the improved resistance parameter factor which was determined based on the vehicle operating speed and the number of units. According to the requirements of EMUs acceleration and traction, the power of 4-18 assembled vehicles with different speeds was calculated, and the power of the traction motor and the converter was calculated in turn. In order to promote the lightweight and integrated technology of traction system, the main converter selected the power device of 3 300 V/500 A SiC hybrid module. The design calculated the ratio between the numbers of motor cars and trailer cars, gearbox speed ratio by contrasting the power of 4-18 marshalling vehicles EMUs with three different speed levels and considering traction motor limits. Traction performance under three fault states was verified under different speed classes and marshalling conditions. The results showed that the designed traction system of the variable-marshalling EMU is feasible and the performance of traction, starting and electric braking was good.