Abstract: The surface temperature of an SS400 beam blank was measured with an CIT-M Infrared Pyrometer and the hot ductility of SS400 steel was tested in a Gleeble-2000 thermomechanical simulator. The results showed that the surface temperatures of the SS400 beam blank in the secondary cooling area and at the straightening places were normally in 760-880℃, i.e., the fragile temperature range of SS400 steel. The rebounds of surface temperature between some supporting rollers in the second segment of secondary cooling area, caused by discontinuously water spraying, were higher than the permitted range 100℃ ·m^-1 according to the metallurgical rules for secondary cooling.