In order to improve the quality of steel strands, mould electromagnetic stirrers (M-EMS) have become a widely used technique to control the molten steel flow in the mould. In this paper, a coupled three-dimensional numerical simulation of the electromagnetic field and flow field in the mould with M-EMS for round billet continuous casting was carried out based on the finite element method. The effects of the current, frequency and the nozzle insertion depth on the flow field are discussed. According to the calculations and analysis, the optimum combination of the current and frequency is 150-250 A and 4-6 Hz, and the corresponding depth of nozzle 100-120 mm. Industrial application shows that with these optimum parameters, the round billet quality was improved effectively and the simulation can serve for actual production.