The present study investigates the output feedback trajectory tracking control strategy for a Human Support Robot (HSR) considering input constraint. A velocity observer is designed to observe the velocity information of HSR, which will be used to the design of output feedback controller. Based on Lyapunov stability theory and LaSalle's invariance principle, it is proved that the designed output feedback controller can make the tracking error system asymptotically stable. Simulation results demonstrate the necessary consideration of the input constraint and the effectiveness of the proposed method.