A mathematical model for numerical simulating the temperature distribution in reheating furnace of hot rolling process is developed by using FEM method.The heat transfer boundary condition of slab is determined by Furnace black box testing,while the validity of the numerical simulation is verified by using the black box testing experimental data.The results indicate that raising the temperatures of preheating section and the slab initial could shorten the slab heating time,guarantee the final quality of slab and improve the reheating furnaces productivity.