This paper considers the posture control of Acrobot system with respect to the input-output linearization with time-varying output. Acrobot system has continuous set of equilibria, and there exists and output function such that corresponding equilibrium is stable via input-output linearization. Posture control from one equilibrium to another is realized by switching two output functions. Proposed method is to connect two outputs with time-varying function. The time-varying function is designed to keep some smooth-simple form and easy to be analyzed.