The low frequency abnormal vibration in light truck system is investigated with experiment and optimization design. On the basis of the vibration testing for light truck, vehicle natural frequencies analytical and optimization design are carried out by using finite element simulation and topology optimization technology. According to the finite element analytical and vibration testing for the vehicle, the low frequency abnormal vibration fact of light truck is discovered. It is found that the dynamic wheel imbalance results in the excitation frequency which is close to the first-order natural frequency of vehicle system, and then the whole vehicle system is resonated. The result shows that the first-order natural frequency of vehicle system is improved by the way of optimizing the frame structure and matching the cab rubber pad stiffness, and the low frequency abnormal vibration of light truck caused by resonance is finally removed.