Abstract: Gas tungsten arc welding (GTAW) inverter system is non-linear one with large signals that is difficult to estimate by small-signal analytic methods. According to control structure of inverters and features of GTAW, a model of closed-loop system for GTAW inverter system has been developed by means of Matlab Simulink. The simulating study has been focused on the adjusting parameters of proportional integral (PI). The research indicated that the ratio coefficient of PI controller has a great effect on dynamic response of the system and its integral coefficient affects system's steady-state error. Inproperly electing the two parameters will make the system unstability.