Numerical method of singular porturbation is applied to casting process.A numerical simulate method of heat transfer and solidification process of slab in concasting mould is given.The mathematical model of heat transfer of slab in concasting mould is attributed to a boundary layer type singular perturbation.By analysing and calculating,the temperature distribution of steel billet in continous casting mould,the width of liquid pool of steel billet and the thickness of solidified shell of the billet are obtained.