In response to the problem of high-frequency oscillation caused by non-load line input in modular multilevel converter (MMC) systems, firstly, a simplified impedance model of MMC at high frequency is established. Based on impedance analysis, the electrical mechanism of high-frequency oscillation of single one MMC with non-load line is analyzed. Then, the modeling of the receiving-end converter station under VF (voltage & frequency) control and droop control is conducted, and the influence of high-frequency oscillation energy on the AC and DC sides of the receiving-end MMC is analyzed. For VF control, the propagation mechanism of oscillation is analyzed from the perspective of the flow path of harmonic signals. The simulation results verify the correctness of the high-frequency oscillation propagation mechanism.