Abstract: Laser fillet welding process of automotive aluminum alloys may have defects of backside quality issues, including back melting and cracking, which affect the appearance and strength of the product. Taking the coverage area of a certain product that is prone to such problems as laser welding process experiments, macroscopic microstructure analysis of metal materials, metallographic testing, composition analysis, and other methods, this study explores the causes and patterns of failure. Study the feasibility of online detection methods for defects such as melt through and cracking through visual measurement experiments. The results indicate that stable penetration can be avoided through appropriate parameter settings, while controlling heat input in high constraint stress areas to prevent back cracking. At high production rates, visual inspection has a high detection rate for melt through monitoring, but more accurate detection methods need to be sought for cracking issues.