The fluctuation of cage rotating speed will lead to the strong instability of the cage movement of the rolling bearing, which will seriously affect the operation performance and service life of the bearing. In this paper, a nonlinear dynamic model of cylindrical roller bearing NJ306 is established considering the dynamic contact relationship between the roller, the ring, and the cage. The variation of cage motion characteristics and its internal mechanism under the uniform, acceleration, deceleration, and harmonic speed fluctuation are discussed respectively, and the effects of angular acceleration, angular deceleration, amplitude, and period of fluctuation on cage stability are analyzed. The results show that the motion stability of the cage is poor under the variable-speed condition, and it weakens first and then increases under the acceleration condition. Different from the acceleration condition, the motion stability of the cage decreases gradually with the increase of time during deceleration and increases at the end of deceleration. Moreover, the increase of angular acceleration and deceleration increases the variation range of cage motion stability. Under the condition of harmonic speed fluctuation, the stability of the cage increases and decreases periodically with time. The increase of fluctuation amplitude will enhance the motion stability of the cage under the condition of simple harmonic speed fluctuation, and the fluctuation period is the opposite. The results of this paper can provide a theoretical basis for the design and failure analysis of cage.