To overcome the problem of the small convergence region of the constrained variable and the knowledge of accurate model using barrier Lyapunov function backstepping control for the output-constrained system, a robust control for the output-constrained system with global convergence is proposed by using the barrier Lyapunov function. This method expands the convergence region to the whole region, while preventing outputs from violating the constriant without steadystate error under the unknown disturbances and model uncertainties. Furthermore, the requirements on the continuity and differentiability of inputs command are relaxed. Simulation on the control of the electromechanical actuator servo system shows that the control plane has characteristics of globally stability, output-constrained, no steady-state error, fast dynamic response, and the robustness to external disturbances.