In order to analyze the effect of spring plank stiffness on the dynamics performances of swing bogie freight car, the spring plank was considered as an elastic beam using finite element method. According to Hook law, the displacement stiffness and rotation stiffness were gotten when spring plank was connected with swing bolsters, different connection stiffness values were obtained by changing spring plank radius. The multi-body system dynamics model of global vehicle was established by multi-body system dynamics theory, based on taking into account each component connection and contact relation of vehicle. Rational radius of spring plank was presented by comparing with the main dynamics performances of vehicle under different spring plank radii. Analysis results indicate that along with the increase of spring plank radius, the critical velocity of vehicle decreases, the lateral riding index of car body increases, and the running stability indices fluctuate, each main dynamics performance of vehicle is better when spring plank radius is 10 m.