Reducing the weight of automobile components is of significant importance in achieving energy efficiency and emission reduction. To meet the performance requirements and specific strength necessary for their intended use, key components must possess superior service performance. Due to the advantages of light weight, high strength and easy processing, aluminum alloy is always used in the processing and manufacturing of automobile parts, such as steering knuckle, control arm, wheel hub, etc. Taking the steering knuckle part as an example, due to its complex structure, it is difficult to meet the dual requirements of manufacturing accuracy and service strength by using traditional processing technology, but additive manufacturing can solve this problem effectively. In this paper, the A357 aluminum alloy was prepared by selective laser melting, and its microstructure, mechanical properties and fatigue failure mechanism were studied in detail. The results show that the microstructure of A357 aluminum alloy is equiaxed and columnar grains with an alternate distribution. The tensile strength of the alloy is 337-351 MPa, and the elongation after fracture is 10.3%-11.0%. As the fatigue load decreases, the number of cyclic load applications gradually increases. Under the stress state of 100 MPa, the number of cyclic load applications can reach as high as 3.08×106 times.