Through studying the microstructure of AZ61 alloy solidified in different external fields, it is found that when solidified in single magnetostatic field , the grain boundaries were composed of matrix Mg and meshy Mg-Al-Zn compounds. When solidified in the field composed of magnetostatic field and alternating current , the grain boundaries were composed of matrix Mg and Mg-Al-Zn compounds which existed as isolated spots as well as a certain amount in the meshy state. But when solidified in the field composed of magnetostatic field and direct current, the mesh disappeared and together with matrix Mg a little Mg-Al-Zn compound appeared as discontinuous flakes. Compared with single magnetostatic field, solidification in magnetostatic field and currenmakes the solute content in grains to increase and the solute content in grain boundaries to decrease.