Abstract: Time delay is one of the characteristics of crack initiation and crack propagation of welding cold cracking, which makes it more difficult to detect the cold cracking. In this paper, acoustic emission testing technique was used for the detection experiments on the natural cooling stage after welding for the inclined yshaped groove butt welding specimens, which were of different thickness and different restraint welding lengths. It can accurately distinguish the cold cracking initiation point according to hitstime and amplitudetime chart of acoustic emission parameters. The analysis result shows that crack initiation and crack propagation of welding cold cracking has a typical characteristic, which can be described as “one point four courses”, including the welding process，birth process, crack initiation point, microcracks convergence process and main cracking propagation process.