Abstract: The algorithm to extractthe plasma arc welding pool edge was studied in this paper. Large area of the saturated region formed in weld poolimages because of reflected arc by weld pool surface. The gray gradient on the boundary of arc region is greater than that of the weld pool. So weld pool boundary can not accurately be extracted by the traditional image algorithms. Aiming at this problem, a multi-parameters constraint algorithm was proposed. The edge information of n frames processed weld pool images and arc minimum circumscribed curve were used as prior knowledge in the algorithm. The results show that the algorithm can effectively eliminate the pseudo boundary and weld pool edge can be detected well and quickly.