For researching the interrupted mutil-step aging techniques of Mg alloy, the effect T6I6 on microstructures and mechanical properties of the new alloy ZM61 is studied by XRD analysis, optional microscopy, electronic microscopy and EDS,TEM,hardness and machanical properties tests,etc. The results show that the T6I6 techniques, compared with T6, improve alloy strength more notably. The hardness of ZM61 after treated by T6I6 is 90HV, YS is 340MPa, UTS is 356 MPa, and the increased ratio of strength are 13% and 8% respectively. During the first step and second step of T6I6, only Mn continues to precipitate as α-Mn and forms more Mn simple particles. The grain boundaries of ZM61 alloy become clean during T6I6 treatment. The scope of strength increase sharply in third-step aging and the β precipitates distribute dispersively in the matrix while the strengthen effect been exerted drastically.