Abstract: A switching function method analyzes the response of the electrical quantities on both sides of the converter by constructing a mathematical model of the converter. The traditional switching function model, restricted by the impact of the asymmetric conduction characteristics of the converter valve during the AC faults, cannot accurately reflect the harmonic features of the converter. By analyzing the changes of the key electrical quantities such as the commutation voltage phase and the firing angle under the AC fault conditions, the impact of the harmonics on both sides of the converter on the commutation time is emphatically considered, and a conversion that takes into account the interaction of harmonics on the AC and DC sides is given. Based on the method, a switching function model suitable for the asymmetric operating conditions of the converter is established. The CIGRE HVDC standard test model and the ±500kV Niu Cong DC (double-circuit on the same tower) engineering model are used to verify the effectiveness and accuracy of the method.