Abstract: A new designed pantograph’s upper-frame with 5083 aluminum alloy was welded by Tungsten Inert Gas Welding（TIG）.Crack was found in the part connected by upper cross tube and longitudinal pillar in the welding process.In order to find the reason and mechanism of the crack,the samples were analyzed by physical and mechanical testing,metallographic analysis and fracture analysis.The test results shows that the liquefied crack occurs because of the high welding heat input and shrinkage stress,which belongs to welding hot crack.Some the improved method were proposed and verified,the studies can provide the technical support for subsequent trial-manufacture and production of the similar products.