In order to achieve a fast and efficient transportation capacity of China"s railways for large, heavy and valuable goods, CRRC TangShan Co., Ltd. developed a high-speed freight Electric Multiple Units (EMU) equipped with four double-axle bogies. Based on the theory of multi-body system dynamics, this paper established a vehicle dynamics model of the high-speed freight EMU equipped with four double-axle bogies, and studied the influence of primary positioning stiffness on the nonlinear critical speed of the vehicle; At the same time, the influence of primary positioning stiffness on vehicle safety index and stationarity index were studied. The results showed that the nonlinear critical velocity firstly increases and then decreases with the increase of the primary longitudinal positioning stiffness, and monotonically decreases with the increase of the primary lateral positioning stiffness. The lateral stability index of the vehicle increases with the increase of the primary longitudinal positioning stiffness, while the vertical stability index is nearly unchanged. The primary lateral positioning stiffness of the vehicle has no significant effect on the vehicle stability index.