With the continuous casting mold vibration displacement system driven by a servo motor as the research object, in view of the constrained input and states, the system parameter uncertainties and the load torque disturbance, an observer-based adaptive prescribed performance controller is proposed. Firstly, as to the parameter uncertainties and the load torque disturbance, an extended state observer with time-varying gain is designed based on the Lyapunov function, which can preserve the estimation accuracy as well as weakening the peaking phenomenon. The controller is designed by combining the prescribed performance function and the adaptive backstepping technique. Take into account the issue of states and input constraints, the constrained command filters are adopted to solve the problem of "computing inflation". A dynamic compensator is introduced to compensate the error caused by the observer and constrained states, and the stability of the controller is analyzed. Finally, the simulation results show the effectiveness of proposed control scheme.