In order to study the fatigue damage characteristic of low-grade highway bridges in mountainous areas, the vehicle information of Yunnan Provincial Highway S306 Edehe Bridge was counted for 29 days using the weight-in-motion system, the vehicle model and vehicle weight distribution characteristics were analyzed by cluster analysis. Based on the damage accumulation theory and the equivalent damage principle, the load spectrum of a fatigue vehicle of low-grade highways in mountainous areas was formulated, and that was simplified to obtain a standard fatigue vehicle. The results show that 33 types of vehicles can be divided into five categories, the two-axle vehicle is dominant, and the vehicle weights follow a lognormal distribution, and the vehicle weights of the other vehicles follow a three-dimensional Gaussian mixed distribution. The standard fatigue car adopts the four-axle and six-axle models with the largest load contribution, and the load contribution value of both is 61.31%.