Abstract: Hot cracking is a defect appearing easily In ultra-narrow gap welding. By using ultra-narrow gap welding with flux strips constrainting arc test, the effects of welding parameters and gap width on the hot cracking were studied. The result showed that the relationship between aspect factor of weld with heat input and that with gap width are the main factors to determine the hot cracking. At smaller aspect factor of weld and larger heat input, the tendency of hot cracking is larger, and with the decrease of the gap width, the hot cracking tendency significantly increase. When the aspect factor of weld increase to a critical value, the welding hot cracking will not occurred, and the critical value is gradually increased with the increase of heat input, while gradually decreased with the increase of the gap width.