Solidification temperature range, temperature sensitivity of solid fraction, temperature process window and phase composition and transformation path of Mg-9Zn-xAl(x=2,4,6) were calculated by Pandat calculation, and studied by experiment. The results indicate that the bigger temperature window is conducive to the control and operation of semi-solid forming for Mg-9Zn-xAl alloy, and the temperature sensitivity of solid fraction is all smaller than 0.015. Mg-9Zn-xAl alloy has the different transformation temperature and path of the phase at the different Al addition. α-Mg, MgZn, Mg32(Al,Zn)49 and Al5Mg11Zn4 were formed in all samples, when Al addition is increased to 6%, Mg17Al12 phase was formed. The grain size and roundness of Mg-9Zn-xAl alloys which prepared by self-inoculation rheo-diecasting method are 65.3μm, 56.5μm, 52.2μm and 1.3, 1.19, 1.23 respectively. When the operating temperature is 150℃, the comprehensive strength and deformation are 278MPa, 283MPa, 295MPa and 36.22%, 33.02%, 31.21% respectively.