Abstract: To study full digital controlled system for CO2 short circuit, the change of droplet volume in arcing period and the behavior of liquid bridge in short circuiting period were analyzed, and thus the mathematic model of the dynamic behavior of the droplet behavior and liquid bridge was built. A mathematic model was also established to describe dynamic voltage loading for short circuit transfer and dynamic processes of welding arc length. With Matlab / Simulink, the simulation model of full digital controlled welding system model for CO₂ short circuit welding was built. The experiment results validate the developed simulation model, and it can be applied to the research of digital regulator rules and dynamic behavior of CO₂ short circuiting welding with full digital controlled system.