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 becomes 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 presented. This procedure takes the gain requirement, soft-switching condition and efficiency of the converter into account. An example with the power of 1 kW and output voltage of 48 Vdc is built to verify the procedure of parameter design for the single-stage AC/DC LLC converter.