With the increase of power electronic equipment in the power grid, harmonics and their hazards gradually appear, and have drawn more and more attention. As converters are the main harmonic sources in DC transmission system, it seems particularly important to further analyze them on harmonics. Taking the line commutated converter high voltage direct current (LCC-HVDC) system as the objective, the paper uses an harmonic calculation model based on the switching function to analyze characteristic harmonics. Firs-tly, based on the modulation theory, AC-DC harmonic calculation equations considering the switching function are established, while the switching functions of voltage and current are solved based on the working principle of the converter. Then the equivalent harmonic impedance model of DC system is obtained by structure analysis of DC system. Based on the above analysis and flow analysis on the AC side, the harmonic equations are solved to obtain the LCC characteristic harmonics of AC and DC sides. Finally, by comparing with the PSCAD simulation model, the feasible and accurate of harmonic calculation method are verified. The paper can provide theoretical basis and method reference for power system harmonic analysis, harmonic suppression and so on.