Considering that the flight control of unmanned combat aerial vehicle (UCAV) is required to have high accuracy, high attacking angle and rapid change of flight state during UCAVs autonomous attacking, we designed a nonlinear backstepping flight controller.Firstly, we established a nonlinear flight model based on six degree-of-freedom aircraft equation.Then, we separated the speed controller design from the attitude controller design based on singular perturbation theory and backstepping control.The problem of ‘differential explosion’ in traditional backstepping was overcome by using simple first order command filter.The tracking error of the controller was proved to converge exponentially to zero according to Lyapunov stability theory.The simulation result proves that the nonlinear controller can track the input reference signal with high accuracy even when it changes dramatically.