For the sliding mode(SM) controlled Buck converters, this paper investigates the harmonic influence of Hall sensor's unmodeled dynamics and the nonlinearity of the SM controller. Based on the model of Buck converters, the system's stability is analyzed, and the parameter tuning of the traditional SM controller is carried out on the basis of the Lyapunov stability theory. On the premise of the ignored sensor, the system's stability is analyzed using the describing function method and the Nyquist stability criterion. Considering the characteristics of the Hall sensor, the model of the Buck converter-sensor is established. Restricted by the system critical stability, the constraint condition is deduced, proving the existence of harmonics induced by the Hall sensor. Meanwhile, the relationship of the SM parameter and the sensor's parameter with the amplitude and frequency of the outputted harmonics is investigated. Simulation results show the correctness and effectiveness of the proposed approach.