Vehicle system dynamic models of bogie driven directly by permanent-magnet synchronous motor were established based on SIMPACK. With the simulation calculation, the structure and main parameters were selected for the primary suspension system of directdrive bogie. The relationship between the non-linear critical speed and horizontal stiffness of the primary suspension system was analyzed, and the vertical and horizontal stiffness was obtained as 10 MN/m and 6 MN/m, which indicated that the primary suspension should apply double-ring steel spring & triangular rod structure. According to the short span of the primary suspension system, analysis of vehicle flexibility coefficient was focused on, and the vertical stiffness value of 1.2~1.6 MN/m was obtained, then, spring parameters were finally calculated.