Abstract: The porosity, segregation and cracks of continuous casting (CC) alloy steel billets in solidification process were investigated in situ, of which the 200 mm × 200 mm billets of GCrl5 (for use of bearings), 40Cr and 20CrMnTi were taken as specimens. A mathematical model of heat transfer in solidification process is therefore set up, with boundary condition, initial condition, surface heat flux at different cooling sections and physical parameters of different steel grades all determined. In particular the 2-dimensional coordinate system is set up on the billets, thus solving dynamically the problem of temperature field on the whole sectional area of billet in solidification process at different overheated temperatures, drawing speeds, times required and positions. The model shows a fairly good agreement with the measurements in situ as verified through numerical calculation. It will provide a reference for taking measures to improve the quality of billets, such as, adjusting technological parameters in continuous casting process, positioning the electromagnetic stirrer at the final solidification area for billets and determining electromagnetic parameters.