The invention discloses an ageing strengthening magnesium alloy with evenly dispersed and distributed granular quasi crystals and rod-like phases. The ageing strengthening magnesium alloy is an Mg-Zn-Y alloy, wherein the content of Mg ranges from 89 wt.% to 93 wt.%, the content of Zn ranges from 6 wt.% to 8 wt.%, and the content of Y ranges from 1 wt.% to 3 wt.%. The invention further discloses a preparing method of the ageing strengthening magnesium alloy. The ageing strengthening magnesium alloy with the evenly dispersed and distributed granular quasi crystals and rod-like phases beta 1' is obtained in the manner that on the basis of preparing of the cast-state Mg-Zn-Y alloy, the solid solution treatment and ageing treatment under the effect of high pressure are conducted. The high-density granular quasi crystals with the size being about 10 nm and the beta1-MgZn2 precipitation phases are evenly dispersed and distributed on a magnesium base body, and therefore the alloy has high peak value ageing hardness during ageing at the temperature being 200 DEG C and the hardness reaches 105 HV. The alloy can be subjected to long-term ageing treatment under the condition of the temperature being 200 DEG C, and good heat stability is achieved. The preparing method is simple in step and easy to operate, and conditions are easy to control and achieve.