In this paper, an appointed time active vibration suppression controller is studied for flexible satellite with piezoelectric actuators. Firstly, a piecewise sliding mode controller with the appointed time is designed to guarantee that the attitude error converges at the appointed time. The convergence time of the model can be set with a prior parameter. Secondly, a modal observer is used to estimate the vibration of the flexible elastic modal, and an active vibration suppression controller is designed to converge at a predefined time. The stability of the two controllers is demonstrated by a Lyapunov theory. The Simulink simulation framework is eventually established, and the numerical simulation results of various data demonstrate the appointed time convergence of the proposed controller. Compared with adaptive sliding mode and nonsingular terminal sliding mode, the controller can achieve higher precision control with less energy consumption.