Abstract: The micro porosity is one of the common defects in aluminum alloy gearbox. which has an adverse impact on the safe service of the gearbox. In this paper, a finite element model of micro porosity is built based on the three-dimensional representative volume element, and the effects of micro porosity on the elastic modulus, yield strength and fatigue strength of aluminum alloy were investigated. Then, a porosity in the gearbox was measured by microphotography and the local fatigue strength was evaluated according to the mechanical properties of materials in the micro porosity area. The results show that the equivalent elastic modulus, yield strength and fatigue strength of AlSi7Mg decrease significantly with the increase of porosity. When the porosity is 10%, the yield strength of the material decreases from 195.5 MPa to 157.0 MPa, and the local fatigue safety factor of the aluminum alloy box decreases by 15.6%.