A novel direct adaptive fuzzy backstepping control approach is presented for a class of output error constrained nonlinear system with input saturation feature. A Butterworth low pass filter and a fuzzy state observer are designed to estimate the unmeasured states of the system which control directions are unknown. To address the output error constraint, a barrier Lyapunov function is employed. Then the smoothness properties of a hyperbolic tangent function and a Nussbaum gain function are used to handle the input saturation. A robust controller is designed combining the backstepping approach with the dynamic surface method. It is proved with the Lyapunov analysis that the proposed control scheme can guarantee that all the signals of the closed-loop system are Semi-Globally Uniformly and Ultimately Bounded (SGUUB). Simulation is made by taking the longitudinal dynamics with parameter uncertainty and external disturbances as the model. The simulation results show that the output error is limited in the constrained range, and the proposed control approach has strong robustness.