In this paper, modal analysis of rigid-vehicle-loaded girder bridge vibration based on perturbation method is presented. The governing differential equation for the girder bridge free vibration analysis is established by introducing a dimensionless parameter, the relative mass, which is the ratio between the masses of the rigid vehicle and girder bridge. The differential equations at the zero-th and the first order scales are expressed and the first order perturbation terms of the natural frequencies and mode shapes are solved. Case studies on a clamped-clamped girder bridge verify the accuracy of the derivation. The results show that the inertial effect of the rigid-vehicle will induce the maximum variation when the vehicle is located at the peak points of the mode shapes and no variation if the vehicle is located at the inflection points.