Resonant DC-DC converters are widely used as power electronic transformers, and their operating characteristics have received much attention in order to better study their dynamic processes. This paper analyzes the steady-state and transient-state processes of resonant converters, presents a DC terminal dynamic model of resonant DC-DC converters. Then, using a clarify and rigorous way derives it, which compensates for the shortage of past studies. Finally, verifying the derivation of the paper by simulation.