Conventional method is getting static displacement calculated by dynamic displacement and impact coefficient aim at making the detection of the bridge faster and more precise. But in fact, it is too difficult to ensure the dynamic displacement under the influence of roughness of the bridge deck. For the reasons given above, this paper simulates the bumping testing on the cross section of midspan by explicit dynamic FEM. A vehicle passes through the cross tie which is higher than the highest unevenness, and then gives an impact load to the cross section of the midspan for getting the maximum dynamic displacement of the cross section. This method can eliminate the influence of roughness of the bridge deck and it is a new pathway of bridge dynamic testing.