To study the vibration characters of the bridge in-service, the impact factor of bridge structures under random traffic loads was presented. The method of influence surface was firstly used to simulate the functions, which was used to obtain the max-value of static displacements by solving the max-value of the function. Secondly, based on each three-dimensional vehicle model and the program of the vehicle-bridge coupled system, the impact forces between each vehicle and the bridge were obtained and then used instead of the whole three-dimensional vehicles in the simulating random traffic loads. And finally, a new method of calculating the impact factor under random traffic loads was obtained and verified. Compared with the impact factor obtained by the Chinese bridge standard and the above method, the results have shown that the proposed method can effectively calculate the impact factor of bridge structures under random traffic.