Abstract: Using welding thermal simulation, the effect of weld-ing thermal cycle on impact toughness was investigated by means of charpy impart test and metalographic test, whose specimens were samplinged from surface and center of continuous casting 10CrNi3MoV steel. The results show that center segregation deterio-rates the impact toughenss of center part and the parameter of weld-ing thermal cycle has different effect on toughenss of steel. The rea-sons for physical and metallurgical factor are the element segregation of C and S and the difference in structure and grain size which has subjected to different welding thermal cycle.