Abstract: Cracks were found in 6061 aluminum alloy corner joint. The crack was identified as liquation crack according to the distribution,microstructure and fracture morphology of the crack. The analysis showed that there were three important causes of the crack, coarse grain of the material, high welding heat input and weld shape deviates upward caused by magnetic blow. The problem of liquation crack of the joint was solved by selecting material with fine metal, reducing welding heat input and improving weld shape.