Abstract: In order to evaluate the voltage quality of direct current (DC) side in hybrid high voltage direct current (HVDC) transmission, the harmonic component and amplitude calculation equivalent circuit of the DC side in HVDC are analyzed. The average switching state method is used to analyze modular multilevel converter (MMC) and the modulation theory is applied to analyze the harmonics generated by line commutated converter (LCC). Then, amplitude calculation expression and simplified equivalent circuit of each harmonic are derived. On this basis, the LC filtering waveform and harmonic components are further examined and the result shows that the LC filter has poor effect in suppressing the low-frequency harmonics under the large alternating current (AC) three-phase unbalance factor. Thus two kinds of third-order low-pass filters are designed to suppress low frequency fluctuation of DC side caused by three-phase unbalance. Comparing the difference of the filtering effect and the dynamic response time between the designed filters and LC filter, the results show that the Butterworth filter has the best index. The hybrid HVDC transmission model is established by using MATLAB/Simulink software. The simulation results show that the derived harmonic amplitude calculation expression and equivalent circuit can correctly reflect the harmonic characteristics of the DC side, and the designed filters has better effect in eliminating the low frequency harmonics.