For increasing heat efficiency and preventing crack resulting in thermal stress, the laws of heating slab must be accurately studied. The thermal stress field distribution in slab heating process is researched, which adopts FEM and bases on the transient non-linear heat analysis theory. The 3500 slab heating model is established in according to the heating technique in mills. The factor of conduction coefficient and specific heat changing with temperature also are considered. The heating process is simulated and the influence laws of furnace temperature and heating rate are drawn, which is very significant in engineering.