In the bridge design code of China, there is no explicit provision on the extreme load combination coefficient. In the probabilistic limit state design method based on structural reliability, due to the fact that there are many kinds of functions of highway bridges and the most of change with time, the combination of various functions is relatively complex, and it is necessary to select a reasonable probability model to ensure that the true and reasonable structural reliability can be calculated. The probability distribution model of permanent load and effect of the highway bridge is established by consulting the literatures and combining with the vehicle data analysis of WIM system statistics; Based on the MATLAB software, on the basis that the overall vehicle weight obeys the multimodal distribution, the measured data are trained and fitted, the probability model of vehicle load effect is established, and the classification loading mode of vehicle load is determined, which lays a foundation for the reliability research of highway bridges and the solution of extreme load combination coefficient.