Collisions between over-high trucks and protection frame for height limit happen frequently, which seriously destroyed the railway infrastructure and affected the road traffic safety. Based on nonlinear software ANSYS/LS-DYNA, the collision process of over-high trucks and protection frame is simulated, and the failure mechanism of protection frame is analyzed respectively from deformation, stress, load etc. Load analysis results indicate that the maximum impact force is approximately direct proportional to the initial velocity, which can be used for choosing the collision load in the design. Stress analysis results indicate that the maximum stress usually occurs in the protection frame crossbar and joint area. Adding the diagonal bracing or increasing the crossbar thickness are the effective methods to improve the impact resistance capability of the protection frame.