The as-cast AZ91 Mg alloy samples were cryogenic treated with different time. Otherwise, optical microscope (OM), mechanical test, resistance test and XRD analysis were used to research the microstructure evolution and physical variation of the samples before and after cryogenic treatment (CT). Due to CT, the structure of as-cast AZ91 Mg alloy was changed from disordered solid solution to ordered solid solution. Firstly, the appearance of ordered solid solution leads to the improvement of peak stress after CT, because of ordered strengthening. Secondly, resistance and crystallographic lattice constant of the samples reduce obviously. Otherwise, the frame-type twinning which is created from matrix in the cryogenic environment could hinder the twin growth and cause the ordered strengthening. Otherwise, optical microscope (OM), mechanical test, resistance test and XRD analysis were used to research the microstructure evolution and physical variation of the samples before and after cryogenic treatment ( Due to CT, the structure of as-cast AZ91 Mg alloy was changed from disordered solid solution to ordered solid solution. First, the appearance of ordered solid solution leads to the improvement of peak stress after CT, because of ordered strengthening solution. Secondly, resistance and crystallographic lattice constant of the samples reduce obviously. Otherwise, the frame-type twinning which is created from matrix in the cryogenic environment could hinder the twin growth and cause the ordered strengthening.