A methane pump shell is formed by selective laser melting of AlSi10Mg alloy powder, namely 3D printing rapid prototyping technology. But the methane pump shell is found to have interlayer cracks running through the contour after forming. The chemical composition of the material was detected by spectrograph and inductively coupled plasma emission spectrometer(ICP), the mechanical properties was tested by universal testing machine, the macroscopic morphology of the crack was analyzed by stereomicrograph, the microstructure of the fracture was analyzed by field emission scanning electron microscope, and the microstructure of the crack and the matrix was detected by the material microscope. The results show that the crack on the inner cavity surface of AlSi10Mg methane pump shell is actually the defect of interlayer unfusion during laser forming of the aluminum alloy shell. The reason is that the laser oscillator instability lead to low power and scanning speed is too fast.