In this paper, an adaptive fixed time back-stepping control strategy is proposed for an uncertain nonlinear system with asymmetric output constraints and external disturbances. Firstly, an asymmetric barrier Lyapunov function is used to deal with the output constraints of the system. Then, a fixed-time disturbance observer is designed to estimate external disturbances. An adaptive fixed-time filter is designed to solve the problem of “term of explosion” in traditional back-stepping control. An adaptive law is used to estimate the unknown upper bounds of the time derivative of the virtual control input. Moreover, based on the Lyapunov stability theory, it is proved that the bounded stability of the closed-loop system is achieved in fixed time and the output satisfies the constraint conditions. Finally, the effectiveness of the control strategy is verified by an application of a permanent magnet synchronous motor system.