In order to adapt to the development trend of high frequency and high power density of converter products, the working principle of LLC resonant soft-switching circuit was mainly introduced, the main mathematical relationship of LLC resonant network was studied, and the theoretical conditions of soft-switching realization were analyzed. Three constraints of LLC resonant soft-switching circuit, including the time constraints of junction capacitance discharge, zero-crossing constraints of primary current and frequency matching constraints, were proposed. On this basis, a 60 kW DC/DC converter based on LLC resonant soft-switching was built, and the feasibility of the design was verified by experiments.