To measure the impact load of moving vehicles on bridge expansion joint and avoid the early damage of bridge expansion joint under such impact, models of vehicle and bridge were constructed with a scale of 1/30, and the experimental study of the model vehicle passing through the model bridge was carried out. Considering the change of the contact length between the tire and the road when the wheel passes through the bridge expansion joint, two improved calculation methods for the impact load on the expansion joint were proposed, namely improved calculation method with hypothetical trajectory of tire bottom and improved calculation method based on the distributed spring-damper element. Then the numerical calculation was compared with the experimental results of the scale model. Finally, based on the improved distributed spring-damper element, the numerical simulation and parameter analysis of the impact load induced by three-axle heavy vehicle at single-gap expansion joint were carried out. Results show that compared with the previous method, the impact load of the expansion joint obtained by the improved calculation methods was closer to the measured value. Therefore, it is necessary to consider the change of the contact length between the tire and the road for the calculation of impact load. The maximum impact factor of the wheel load of heavy vehicle exceeded the allowable value of the bridge design specification, which can be controlled within the allowable range by adjusting the opening width of the expansion joint.