Using different electromagnetic fields application ways, AZ31 magnesium alloy was semi-continuously cast into billets with diameter of 200 mm. The effects of the different electromagnetic fields application ways on the microstructures and mechanical properties of AZ31 billets were investigated. The results show that compared with the conventional direct-chill semi-continuous cast billets, the microstructures in the entire cross section of the AZ31 billets cast in the single alternating magnetic field or the combination of alternating magnetic field and stationary magnetic field are significantly refined, the β-Mg17Al12 phases become fine and dispersive, the difference of the grain size between the border and center of the billet is reduced, and the difference of the microstructures in the billet cast in the combination of alternating magnetic field and stationary magnetic field is smaller than that of the single alternating magnetic field casting. The two electromagnetic field application ways are all beneficial to the homogeneous distributing of the alloy main elements in the cast billets, to great suppressing of macro-segregation and to improving of the mechanical properties of the billets. Compared with the properties of the conventional cast billets, the yield strength and the ultimate tensile strength of the billet cast in the electromagnetic field are increased by 50~60 MPa and 40~50 MPa, respectively, and the elongation is doubled.