The contents of tramp elements Cu, As and Sn as well as P in A105 continuous casting billets were analyzed by field emission scanning electron microscopy ( FESEM) equipped with energy dispersive spectrometry ( EDS) . Main precipitates in the steel and the variation of P content in molten steel with solid fraction were calculated by Thermo-Calc thermodynamic software. The hot ductility of the steel was measured by using a Gleeble 1500 thermal simulator. It is found that P segregation is the main reason for transverse cracking of the steel. The segregation of tramp elements Cu, As and Sn at grain boundaries promotes the formation of cracks. Straightening in a lower temperature zone accelerates crack propagation. AlN precipitation may have nothing to do with the formation of cracks due to little AlN.