Abstract: The algorithm of threshold automatic selection based on fuzzy reasoning for image processing of arc-v-shape weld seam is introduced. The removal method of the strong arc interference is proposed based on path tracking. By designing nine kinds of window templates, and acquiring the maximum average gray value in the region where these templates can cover through column scanning mode to get the vertical positions where the average gray value are maximum, the threshold is obtained through fuzzy reasoning using the maximum gray value and its increment. After that, all of the positions are tracked to find the white spots below the lowest one and above the tiptop one, and then eliminate the disturbances formed by blebs or little strong arcs below the lowest position and upon the tiptop.