For the cracking problem of aluminum alloy shell during rough machining, the cracking shell sample was detected and analyzed by metallographic microscope, scanning electron microscope and energy dispersive spectrometer. After precision forging of aluminum alloy shell, the surface of forgings shows the forging streamline with uneven deformation, and the deformation amount in the fillet part of forgings was the largest. And, there is a surface crack of bending deformation on the surface layer of precision forging sample, which is supposed to be a folded forging crack. Then after rough machining of aluminum alloy shell, cracks occurr along the rounded bottom of shell with the largest deformation amount. Furthermore, the surface crack growth characteristics of cracked sample are curved arcs, and there are fiber organization and pickling residual corrosion pits in the crack, so it is inferred that the surface cracks belong to hot forging cracks. In addition, there are claw-shaped extrusion cracks at end of the cracks caused by insufficient plasticity, indicating that the temperature of sample surface is low. The results of energy spectrum test show that the oxygen content in the crack is as high as 23.35%, indicating that the surface crack is oxidized at high temperature, and it is further verified that the surface crack is generated during the forging process. The main reasons for the cracking of aluminum alloy shell are obtained by the analysis that the surface temperature of forgings during the forging process is law and the plasticity of material is insufficient, the deformations in the surface and subsurface of forgings are inconsistent, the forging folding crack appears at the counded bottom with the largest deformation amount of forgings, and the surface cracks reclose due to the continuous deformation during the forging process, but the surface crack reopens due to the stress release during the rough machining process.