Abstract: Inclusion is an important factor affecting the mechanical properties of ZM6 magnesium alloy castings. The mechanical properties of ZM6 magnesium alloy with inclusion defects were tested by tensile tests, and the macroscopic and microscopic morphology of the fracture samples were observed. The size of inclusions was measured, and the effect of the equivalent diameter of inclusions on tensile strength and elongation was quantitatively studied. The functional relationship between the tensile strength and elongation and the equivalent diameter of inclusions was established. Finite models of ZM6 magnesium alloys containing different inclusions were established by ANSYS software. The results show that the inclusion of ZM6 alloy is mainly magnesium oxide. The equivalent diameter can represent the inclusion size well. The tensile strength and elongation will decrease with the increase of the equivalent diameter of inclusion. The finite element simulation results show that there is obvious stress concentration at the interface between inclusions and matrix, and it is located at the thin interface of matrix. The tensile properties of alloys containing inclusions of different sizes can be predicted accurately. The deviation of tensile stress-strain curve is within 5%, and the error of tensile strength is less than 10%.