Abstract: The AC-DC coupling occur more and more frequently in the high voltage direct current (HVDC) transmission system. When the AC system at the rectifier side fails, the improper response of the DC control system responds may further cause the commutation failure of the inverter. In this paper, the response process of the DC control system and the variation characteristics of the commutation voltage at the inverter side are analyzed firstly after the AC system at rectifier side fails. Then, combined with the calculation expression of the extinction angle, the reasons for the decline of the extinction angle during the fault recovery process are discussed. The analysis shows that the commutation voltage of the inverter side varies in a small range under the fault of the AC system at the rectifier side. The main cause of the commutation failure is the rapid rise of the DC voltage and the DC current during the fault recovery process. On this basis, the method to slow down the recovery speed of DC voltage by improving the trigger angle at the rectifier side is proposed, and the other method to slow down the recovery speed of the DC current by reducing the current margin and improving the current order value at the rectifier side is put forward. Finally, based on the CIGRE HVDC standard model, the effectiveness of the proposed commutation failure suppression measures is verified on the PSCAD/EMTDC electromagnetic transient simulation platform. The simulation results show that both the two methods are able to suppress the commutation failures under the fault of AC system at the rectifier side, and the effect is better when they work together.