Based on the weigh in motion data at a toll station on the Beijing-Zhuhai Expressway, the statistic analysis of vehicle load data was carried out, and Monte Carlo method was used to generate random traffic flow, the maximum bending moment of 30m and 40m simple supported beams were obtained, the extreme-I distribution was used as the bottom fitting and extrapolation, and the predicted value of the maximum bending moment of the simple supported beams were obtained. By comparison with vehicle load effect according to current specification, it provided some reference for the bridge load standard in our country.