The molten pool images of GMAW (Gas Metal-Arc Welding) was acquired during the base time period using a self-built system for measuring the transient temperature field of molten pool. The transient temperature field of the molten pool of GMAW at different welding speeds was calculated based on the principle of colorimetric temperature measurement. When analyzing the temperature distribution at the middle and tail region of the molten pool in the process of hump bead formation under high speed GMAW, it was found that the temperature distribution of the molten pool is uneven perpendicular to the welding direction, and this law does not accord with the time when the hump appears in the image, and the accuracy rate accorded with the law was also counted. Through this rule, the welding robot can adjust and control the process in time before the appearance of hump, and the prediction of hump before bead formation under GMAW is preliminarily realized.