Abstract: In pulsed GTAW with wire filler, the variation of the weld pool shape could be observed from the weld pool image, such as the pool width, length, rear angles, surface height, etc. A skilled welder can adjust the welding parameters according to the variation of the weld pool shape, for ensuring the stability of the bead shape. With the captu red images of the weld pool, the shape parameters of the weld pool can be calculated using image processing algorithm, for fulfill the requirement of real time control. Aiming at the characteristic of the weld pool shape during pulsed GTAW with wire filler, this paper proposed a nonlinear regressive formula for descuribing the weld pool shape with a few shape parameters. A fast algorithm of image processing was designed for extracting the shape parameters of the weld pool in real time. The testing results indicated the accuracy of the nonlinear regressive formula, and verified the feasibility of the image processing algorithm. This laid the foundation of the further modeling and control for the welding process.