This paper proposed an approach to transferring nonlinear parasitic drain-source capacitance of MOSFET to a linear model by charge-related equivalent conversion. It is applied to a previously researched analytical model of the class-D inverter to add its feasibility in high frequency. Based on the proposed approach, the relationship between maximum operation frequency, the nonlinear capacitance, and the input voltage are inferred and verified by spice simulation, resulting in good consistency.