Abstract: The hybrid dual-infeed HVDC system presents different high-frequency oscillation characteristics with different tie-line lengths, due to the existence of line commutated converter based high voltage direct current (LCC-HVDC) system. To study the high-frequency oscillation characteristics and mechanism of the hybrid dual-infeed HVDC system, the state-space model of the system is established, and the high-frequency oscillation characteristics of the system are studied. The results show that the hybrid dual-infeed HVDC system presents different high-frequency oscillation characteristics under different tie-line lengths due to the existence of LCC subsystem. Then, a high-frequency impedance model of the hybrid dual-infeed system is established, and the effects of the LCC subsystem, tie-line length, MMC time-delay and AC line length on the system high-frequency impedances are studied. The results show that for the hybrid dual-infeed HVDC system without electrical proximity, the existence of the LCC AC filter eliminates the risk of high-frequency oscillation, and for the hybrid system with electrical proximity, the suppression effect of LCC AC filter on high-frequency oscillation decreases with the increase of tie-line length. Finally, a method to determine the critical length of tie-line without the risk of high-frequency oscillation for the hybrid dual-infeed HVDC system is proposed, and the effectiveness is verified by the theoretical analysis and electromagnetic transient simulation.