Using different external fields application ways, AZ61 magnesium alloy was semi-continuously cast into billets with diameter of 165mm. The effects of the different external fields application ways on the grain size and hardness of AZ61 billets were investigated. The results show that compared with the conventional direct-chill semi-continuous cast billets, the grain size of the AZ61 billets cast reduced from 248μm in the edge and 276μm in the centre to 196μm and 259μm respectively(with a reduce of 21% and 6.2% respectively) by low-frequency electromagnetic field (f=30Hz, I=60A)，the structure in edge and centre refining effects are significantly diverse. However, the grain size in the edge and centre from reduced from 248μm in the edge and 276μm in the centre to 185μm and 208μm respectively (with a reduce of 25.4% and 24.6% respectively) by combining ultrasonic and LFEC, and the structure in centre refining effects were significantly improved. Moreover, compared with the conventional DC casting, the compound physical Field can improve the hardness of the ingot and obviously reduce the different hardness from edge to centre. Therefore, the compound physical Field obviously improved the inner qualities of AZ61 magnesium alloy ingot billet, and the microstructure is markedly refined, so the hardness of ingot uniformly improved.