With the development of Chinese AC/DC power grid， the voltage stability of commutation bus is urgent to be researched. Firstly， the simple model of AC/DC parallel system is given， and then based on this model the AC system strength index is introduced as well as the load margin graph and voltage stability critical graph. Taking example for the modified two-area and four-generator power system， this paper accurately calculates the node power margin according to the load admittance model algorithm， and discusses the influence of parallel AC system strength and equivalent impedance of receiver system on voltage stability at inverter commutation buses. Finally， through the comparative analysis of load margins under different HVDC steady control modes， it is shown that when the receiver AC system is weak， the control mode of constant DC current at rectifier and constant DC voltage at inverter side will be propitious to improve the system stability.