By considering the influence of the pulley’s inertial mass on the longitudinal vibration of the belt, the belt is simplified as an elastic body with uniformly distributed mass, then the mechanical and the mathematical model of the longitudinal vibration of the belt are established. By taking into consideration of the relative motion between the belt and the pulley, a computing model of the natural frequency and the mode function is built according to the continuous conditions of the belt. The dynamic response of the forced vibration of the system is solved by means of coordinate transformation. The response characteristics of the longitudinal vibration of the belt are analyzed under two excitation forces, one being the pulley speed fluctuation and the other being the load torque fluctuation. The simulation results show that: (1) the response curves for the two excitation forces are both simple harmonic functions and the amplitudes are affected by the inertial mass of the pulley;(2) the instantaneous vibration displacement of the pulley is inversely proportional to its inertial mass. The fluctuation of instantaneous rotational speed of the pulley makes the instantaneous transmission ratio of the belt transmission system unstable, thus leading to the sliding variation loss between the belt and the pulley. The proposed method for the study of the longitudinal vibration characteristics of the belt has great theoretical and practical significance on the design of the belt drive system and analysis of its dynamic characteristics.