Abstract: The speed integrated gear is a new kind of high-efficiency and lightweight transmission component, possessing both of circular and noncircular gear functionalities. Aiming to the special transmission mode, the dynamical characteristics of the new gear pair with the excitation of time-varying instantaneous center was studied. Firstly, the transmission principle of the speed integrated gear was illustrated, getting the change rule of the instantaneous center and the drive ratio equation. Then on the basis of separation of the elastic rotation angle, analyzing time-varying instantaneous center, stiffness, damper and error, the excited mechanism of the instantaneous center on the gear was revealed to build a torsional vibration model. Finally, the vibration feature of the speed integrated gear under different conditions was analyzed based on Runge-Kutta method. The calculation results indicate that the compound excitations of the instantaneous center and the stiffness can lead to complex multi-frequency phenomenon, and the instantaneous center excitation will be more over the stiffness excitation to the torsional vibration with the amplitude increasing of the instantaneous center, becoming the primary cause of the torsional vibration.