Since the LLC topology is widely used in DC-DC converters, this paper firstly analyzes the sufficient and necessary conditions for LLC converter to be used in AC-DC application. Based on the first harmonic approximation method and time domain analysis method, the key parameters design method of LLC for single-stage PFC application is derived and the theoretical expressions are presented, and the effective parameter range is calculated and analyzed to achieve an optimized design. Finally, a single phase 6.6 KW prototype was built. The experimental results show that the prototype can operate in the full input and output voltage range, and the prototype has high efficiency and low total harmonic distortion.