Abstract: The 7075 aluminum alloy cylinder fractured during work, and then was analyzed through chemical composition, room temperature tensile test and fracture morphology to find the fracture reasons, and the relevant improved methods were proposed. The results show that the main reason for the aluminum alloy cylinder fracture was the mechanical property of the alloy decreasing because content of Mg was below the standard content. The micropore on oxide film extended along machining mark to cause fatigue source forning under cyclic stress, the crack extended quickly as the stress circulation intensify, and the fatigue fracture for 7075 aluminum alloy cylinder happened at last.