A composite cylinder leaked during autofrettage. The fracture reason of leakage was found out by fracture， metallographic and stress analysis. The results show that the main reason of fracture and leaage in low pressure is the anisotropic coarse grain structure around the neck of the liner which reduced the toughness， plasticity and strength of the structure. The necking-in heating process and spinning technology is refined， and microstructure around the neck of the liner is improved. Finally， the leakage problem is solved and the quality of composite cylinder is stabilized.