The nonlinear vibration system with slow-changing parameters has been studied. This system has wide background in engineering. In this system the mass, stiffness and exciting force are changed slowly with time, so it is differient from the ordinary time-varied system. In this paper, the analytical method for the slow-changing systems is discussed, and the vibration feature is also studied based on the rotor system with slow-changing stiffness. At last, the way of the controlling the amplitude when the system passes through the resonant region is proposed.