With the increasing of the vehicle speed, traffic volume and moving loads, the dynamic response of bridges under moving loads attracts more and more attentions. If one considers the inertia effect of moving vehicle and the damping effect of bridges, the moving vehicle could be modeled as a moving mass. Its govern equation is a partial differential equation with variable coefficients. The equation is usually difficult to solve exactly in mathematics. Using the variable separation and modal superposition method, the partial differential equation is reduced as an ordinary equation with variable equation set. Using the method of WKB method, an approximate solution of the problem is obtained, which is compared with the results of numerical simulation, the solutions of moving constant forces and Inglis.