We present here a general method for evaluating the steady-state frequency-tracking distortion in the digital Pound-Drever-Hall technique with modulation harmonic distortion. The theoretical tracking distortion model is established based on the multi-beam interference theory. The effects of the additional harmonic phase shift and the relative distortion ratio changes in the model are simulated by the Runge-Kutta method. Moreover, we demonstrate the steady-state frequency-tracking distortion caused by the modulation harmonic distortion in a resonant frequency tracking system with a 35 mm Si3N4 waveguide ring resonator. According to the measured and simulated results, we obtain the optimal modulation frequency and depth with minimal frequency-tracking distortion, which are 11.49 MHz and 3.96, respectively.