Time delay effect is the main factor that causes high frequency oscillation between HVDC transmission system and weak grid. In this paper, a modular multilevel converter (MMC) impedance model fully including typical control structure and its time delay is established. The cause of high frequency oscillation is revealed and verified by simulation model. Then, three common suppression strategies for high frequency oscillation suppression are summarized and compared from the perspective of impedance. Finally, the analysis results show that connecting high-pass filter on the AC bus is a relatively better strategy, and it has a good suppression effect on oscillation in a wide frequency band.