A modular adaptive disturbance rejection backstepping controller is proposed for the nonlinear attitude tracking of satellite in the presence of uncertainties, external disturbances and saturation constraints, which combines the theory of Nonlinear Extended State Observer (NESO) with constrained backstepping.Firstly, the satellite attitude is represented by Modified Rodrigues Parameters (MRP).And the NESO with parameter projection is employed to estimate and compensate the time-varying total disturbances on line for improving the robustness of backstepping controller.Then the backstepping controller is derived by introducing command filters and modified tracking errors to implement any operating constraints and obtain the derivatives of virtual control easily, which can also enlarge the projection sets of the disturbance estimation laws.Lyapunov theory proves that the closed-loop input-to-state stability is guaranteed under the effect of nonlinear damping.Comparative simulations state that the proposed controller has better performance in achieving precise attitude tracking and estimation of disturbance than classical adaptive backstepping.