Aiming at the different response characteristics of multi-terminal HVDC transmission system in the case of AC system faults, this paper introduces the electromechanical transient modeling method of the primary system and control system of multi-terminal HVDC transmission system. A three-terminal DC transmission system with one sending end and two receiving ends is constructed, considering the combination of LCC-LCC-LCC, LCC-VSC-LCC and LCC-VSC-VSC, the corresponding simulation models are established. This paper compares the response characteristics of different multi-terminal DC transmission system in the case of AC system faults with different voltage drops. The comparison results show that the VSC converter on the inverter side can effectively reduce the probability that the DC system cannot perform effective power transmission due to the commutation failure of inverter. The use of VSC converters in both inverters can give full play to the advantages of VSC converters without commutation failure, and maximize the transmission of DC power. At the same time, the full play of the VSC converter requires the close cooperation of the multi-terminal DC transmission control system.