Requirements for increasing power density and service life of vehicle diesel engines lead to fretting wear on the back of main bearing bush of diesel engine due to long-term large alternating load, which have a great effect on the service life of parts and engine. A dynamic load test of 10 million cycles was carried out on the contact pair of main bearing bush and engine block to obtain the state of fretting wear by simulating the actual working load and the influence of different bolt preloads on fretting wear was compared. According to the built simulation model of engine body and bearing bush contact pair and Archard model of fretting wear, the law of fretting wear was analyzed and compared with test result. The results show that the simulation results are in good agreement with the experiments. The most severe wear occurs at the edge of upper bearing bush and increasing the bolt preload leads to the increasing wear of upper bush and the decreasing wear of lower bush. The further analysis of bolt preload and  main bearing interference shows that the location of most severe wear is related to the larger stress and strain caused by the bolt too close to the bearing bush and increasing the interference in a certain range can significantly reduce fretting wear. The results provide suggestions and references for reducing fretting wear of bearings.