In the present work, the principles and processing modes of equal channel angular pressing (ECAP) for as-cast magnesium alloys were investigated. With structural design of ECAP die, the accumulated effect of shear strains under ECAP was calculated. By observation of optical microstructure of single-passed ECAP deformed cast ingot of AM60 magnesium, the effective relationship of die geometric structure (angle and subtending one at corner between entry and exit channels) to the evolution of deformed microstructure was discussed. Based on the curves of displacement versus extrusion force during multi-passed ECAP, a mode of evolution of deformed microstructure was analyzed under different pressing conditions (e.g. number of passes, back-forces, and deforming rate). Routine of experiment and modeling for ECAP of magnesium alloys was schemed. It can be concluded from the study that the ECAP deformed microstructure of AM60 casting ingot fits the one estimated by theory of plastic mechanics well. Multi-passed ECAP can perfect the microstructure of alloy tremendously. For as-cast magnesium alloys characterized by coarse-grained structure, ECAP is an effective thermo-mechanical processing to make their ultra-fine grains.