Abstract: It is significant but difficult to extract a weld in image when the gap of the weld is extremely small.This paper proposed a new method for weld detection, namely the multi-step gray projecting integral (MSGPI) method by circular sub-window.Firstly, the method applies a circular division in image as the sub-window for detection and the means of GPI values based on traditional gray projecting integral (GPI) method, which overcomes shortcoming of the latter one.Secondly, it adopts a link table to store edge points gained by Sobel filtering previously to decrease amount of computation.A threshold method is employed to avoid effectively interference caused by scratching and rust.Thirdly, the searching range of weld's angle by GPI process is divided into two stages:coarse and fine steps, which not only ensures detection accuracy but also greatly decreases amount of computation.The experimental results show that the MSGPI method can achieve rapid and accurate detection for weld even if the gap of weld is quite small.