Taking into account the variation of switching instants， unbalanced commutation overlaps and the nonlinear commutation currents caused by asymmetric faults， a detailed analysis of the dynamic process in the HVDC converter is given. Based on the superposition of basic component， modification component and commutation component， a three-phase voltage and current switching function model and sequence component switching function model of HVDC converter are established. Based on the model， an analysis and calculation method of harmonics in AC/DC system is proposed， in which the effects of harmonic interactions between the AC and DC system and the asymmetric operating conditions of the converter are studied. The mechanism of the harmonic interaction between AC and DC system is revealed by algebraic analytic expression. The method is used to the harmonic analysis and calculation of CIGRE HVDC system and GuiguangⅡ HVDC system. The results are compared with those obtained by simulation using PSCAD/EMTDC software. It is shown that the proposed method is accurate and effective， and provides the analysis basis of harmonic mitigation， filter configuration and protection analysis in AC/DC system.