The single-stage AC/DC converter based on LLC can achieve high compactness due to its zero-voltage switching characteristic. However, due to the nonlinear characteristic of LLC and complex operation mode of the single-stage AC/DC converter, the traditional parameter design methods based on First order Harmonics Approximation (FHA) or State-trajectory analysis method are not suitable. The parameter design of this kind of converters become very complicated. In this paper, a series of parameter design procedure of single-phase single-stage AC/DC LLC converter based on numerical analysis is present. This procedure takes the gain requirement, soft-switching condition and efficiency of the converter into account. An example with the power of 1kW and output voltage of 48Vdc is built to verify the procedure of parameter design for the single-stage AC/DC LLC converter.