Aiming at the output capacitance of FETS, firstly, the nonlinear characteristics of output capacitance are analyzed, the related charac-teristics of output capacitance in soft switching circuit are analyzed, and a fast measurement method of output capacitance of FETS, namely the loss method, is proposed. By designing a step-down soft switching circuit, the accuracy of loss measurement is verified according to the charge change in the process of soft converter, which fully proves the accuracy and rapidness of loss measurement. The difficulty in determining and measuring the output capacitance related characteristics in the design of soft switch circuit is solved suc-cessfully.