Abstract: Macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination and other methods were used to analyze the explosion reason of cylinder tube of an hydraulic cylinder. The results show that the structure of cylinder tube of hydraulic cylinder was uneven and there was mischcrystal, which led to the strong brittleness of the material. Improper welding process led to cracks near the welding arc point, which caused stress concentration. Under the action of large stress, the rapid propagation of cracks led to the explosion of the cylinder.