Abstract: Aiming at a hybrid DC transmission system that uses LCC at the rectifier side and the MMC at the inverter side respectively,the harmonic characteristics of both LCC and MMC are proposed. First,the LCC at the rectifier side is replaced by the 3-pulse harmonic voltage source, whose harmonic output characteristics is equivalent to a 12-pulse converter. Second,the MMC is represented by a passive electric network. At the same time,the corresponding harmonic models of DC transmission lines and DC filters are constructed. Finally,taking the single-pole hybrid DC transmission system as an example,through scanning the impedance-frequency result of the DC loop,the harmonic impedance under different conditions is calculated to judge the resonance situation. The simulation results show that as the line length and the number of filters increases,the resonance frequency decreases.