Abstract: High strength steel is widely used in industrial production because of its excellent properties. However, it is difficult to extract the welding characteristics of high strength steel and evaluate the quality of resistance spot welding. Electrode imprint is a direct reflection of the surface morphology of the spot welding and the end face of the electrode. Imprint image contains both spot welding information and electrode wear information. Through image processing and recognition, the contour edge of imprint image is detected and recognized, and the imprint characteristics parameters are extracted by ellipse fitting method. The imprint characteristic parameters and nugget diameter were analyzed, and the influence of each characteristic on spot welding quality was studied. The results show that the imprint contour fitted by ellipse can approximately replace the actual contour. There is a significant negative correlation between the average axial length and pitting area extracted from imprint characteristics and spot weld quality.