Abstract: By analyzing the short circuit transformation, the force acting on droplets and the control theory of the STT, a mathematical model and a simulation model of the STT dynamic process were established. It analyzes systemically the influence of wire feed speed, melting rate and the variations of droplet on arc length, and founds its mathematical model.This paper also compares the influences of different control mode of the current on the dynamic characteristic of the welding system in the STT periods. Different simulation models were taken into experiment, the simulated results corresponds with the experiment results, which proved the SIM-ULINK models to be correct. The results show that output control mode can reduce the current more rapidly to meet the need of STF control system than invert-loop control mode.