The utility model provides an online uninterrupted power source system based on three -phase four -leg converter, includes main power supply loop, bypass power supply loop, DC power supply return circuit. Main power supply loop is connected with ac bus through the 2nd output switch including the interchange primary input switch, three -phase four -leg rectifier, three -phase four -leg dc -to -ac converter, first static switch, the output switch that draw the connection in proper order a, output switch. Bypass power supply loop is including exchanging bypass input switch, the bypass switch, second static switch, service switch, and service switch and hookup are at the both ends of the branch road of the bypass switch, second static switch, output switch component. The DC power supply returncircuit is including concatenating direct current switch and the dc bus between three -phase four -leg rectifier, three -phase four -leg dc -to -ac converter. The utility model discloses can guarantee output direct current electric energy quality under three -phase AC power supply disequilibrium, nonlinear condition, guarantee output interflow electricity electric energy quality under three -phase AC laod unbalance, nonlinear condition.