The invention provides an on-line voltage stability determination system of an alternating current and direct current mixing connection system. The on-line voltage stability determination system comprises a real-time data collection module, a power transmission line two-node model construction module, an on-line voltage stability evaluation indicator calculation module, and a system instability determination module. The real-time data collection module collects resistances R, reactances X, admittances B, active powers PB of receiving terminals, passive powers QB of receiving terminals and feeding terminal bus voltages UA in real time; the power transmission line two-node model construction module constructs two-node models of the power transmission lines according to the parameters, collected in real time, of the power transmission lines; the on-line voltage stability evaluation indicator calculation module calculates on-line voltage stability evaluation indicators of all power transmission lines based on the two-node models; and the system instability determination module determines whether the system reaches voltage instability boundary according to the on-line voltage stability evaluation indicators of all power transmission lines. According to the provided system, different voltage stability determination methods are used according to different fault properties, thereby improving the calculation speed.