The low frequency electromagnetic casting(LFEC) process was used in casting the large scale AZ31 magnesium slab with the cross section of 300mm×800mm, and its macrostructure and macrosegregation were investigated at different casting velocities. The result shows that the segregation was slight in the centre and become serious in region about 20mm~30mm near the slab surface, There exist three zone in AZ31 magnesium alloy slab, surface chill zone, columnar crystal zone, internal equiaxed crystal zone. With the casting velocity increasing, the size of grain increases first and then decreases, the width of columnar crystal zone decreases drastically as well as the macrosegregation. At high casting velocity, the grain size is finer, but more uneven. Thus, the appropriate casting velocity in this process is 32mm/min.