In order to further improve the efficiency of CLLC converter and reduce the switching loss, synchronous rectification control is often added to the secondary side of the converter. The traditional detection synchronous rectification schemes have high cost, and the results are easily interfered by the circuit parasitic parameters, which limits the application. However, the traditional schemes based on mathematical model are characterized by low accuracy, poor effect or limited to the converter parameters, and poor versatility. In this paper, based on the accurate time-domain model of CLLC converter under the pulse frequency modulation, the tangent approximation combined quadratic function correction method is used to obtain the synchronous rectification angle under under-resonant region and over-resonant region, and the general analytical expression for the synchronous rectification angle of the CLLC converter under variable resonant cavity parameters and load conditions are obtained by surface fitting. Compared with other schemes, this scheme ensures the high-precision solution of synchronous rectification angle without additional detection circuit, and is more versatile. Finally, the feasibility and accuracy of the proposed method are verified by Simulink simulation model.