The microstructures and mechanical properties of AZ91D magnesium alloy produced by two kind of fluxes were analyzed by using mechanical performance testing machine, optical microscope, differential thermal analysis (DTA), X-ray diffraction(XRD), scanning electron microscope（SEM）and energy dispersive spectrometer(EDS) analysis, etc.. The results show that the comprehensive properties of AZ91D magnesium alloy produced by the new flux named NF were better than traditional flux named RJ-2. Phase Mg17Al12 was improved enormously. In addition, not only the nuclear particle A14C3 was formed, but also the grain of the alloy was refined remarkably，and the σb，σs and δ were increased eventually. The tensile fracture surface was made of deeper dimples with rolling patterns, which was relevant to the smaller microstructure.