Innerouter couple cooling technology of molten steel for 1 240 mm×200 mm slab continuous casting is put forward in order to enhance the efficiency of heat transfer and improve inner structure of billet, in which an inner cooling apparatusU shape pipe is set in the mold. Solidification state of molten steel under the coupling condition of flow field and temperature field in innerouter couple cooling mold is simulated by using fluid dynamics software. It is found that inner cooling apparatus can make molten steel flow even, which is favourable to the floating of inclusion in molten steel, increase the solidification rate of billet and improves the quality of billet.