In this paper, the uncertainty and disturbance estimator (UDE)-based control is adopted for a class of nonlinear systems with mismatched uncertainties and input saturation. By integrating with the backstepping approach, the robustness for the mismatched uncertainties is guaranteed by the recursive structure of backstepping. The 'disturbance-like' terms which consist of the unknown dynamics, disturbances and the system intermediate tracking errors are estimated by low-pass filters with the knowledge of spectrum information. An auxiliary system is constructed to deal with the input saturation while only the system order information is known. Stability analysis of the closed-loop system shows that the uniformly ultimate boundedness is guaranteed. A numerical example is provided to illustrate the effectiveness of the proposed method.