Fully DC wind turbine generator system is a promising substitute for conventional AC one. In the system, the LLC step-up converter plays a key role. Mode analysis and time-domain expressions can provide more accurate estimation of the resonant performance than the conventional First Harmonic Analysis method. The circuit works at different modes as the load rate varies. This paper categorizes these resonant behaviors into four modes, with time-domain expressions of each mode obtained. The variation regularity of these modes is given and is applied in the program to realize recognizing the working mode automatically. The simulation and experiment have verified the effectiveness of the proposed method.