Abstract: Aiming at the problems of bad antiinterference capacity and low localization accuracy in traditional localization algorithms of spot center which include the center of gravity method, the Gaussian fitting and so on， we raised a improving way which calculates the center of spot on the basis of the circle fitting method. Using the type of 56 submachine gun, we conducted the single shot experiment and processed the data obtained by testing system through the modified circle fitting, Gaussian fitting, and gravity method, respectively. Compared with the testing result of OFV5000 laser velocimeter，the relative error of improved circle fitting is only 0.92%. The result shows that the algorithm improves the antiinterference ability and achieves the rapid and accurate calculation of spot center，and obtains the weapons motion parameters.