The thickness of solidified shell of strand in secondary cooling zone was first calculated with mathematics model and then confirmed with pinshooting in EAF Steelmaking Plant of Nanjing Iron and Steel Co., Ltd.. The position of crater end 35～55 mm in diameter was found by calculation with modified mathematical model and pinshooting for given casting speed and steel grades and after that the FEMS was fixed at 8 m from meniscus. The optimal parameters for electromagnetic stirring were determined by teslameter:550 A and 13 Hz.The chemical composition at crater and of billets cast with and without FEMS was analysed. The results showed that the carbon and center segregation coeffient of billet cast with FEMS was decreased from 1.09% to 1.01% and the peak sulphur content in center was reduced obviously. The electromagnetic stirring parameters are suitable.