Microstructure and micro-area chemical composition of surface bleeding segregations of magnesium alloy strips processed via twin-roll casting were investigated by OM, SEM and WDS. The results indicate that surface bleeds are composed of solute-rich intermetalics of Al and Zn element, the percentage composition of Mn element in surface bleeds is similar to that of the matrix alloy. The structure of surface bleeds changes into equiaxed grain with fine grain size. Stop-samples were achieved by abruptly interrupting casting for analyzing the solidification and deformation characters in the set-back of magnesium alloys strips, and the formation mechanism of surface bleeds were discussed. Furthermore, the characteristics of solute elements distribution in surface bleedings including Al, Zn and Mn elements were discussed