Abstract: In order to find out the causes for pit defects on the inner surface of the 4130X steel pipe for making of the gas cylinder, the specimen of the pipe is tested in terms of chemical composition, mechanical property and nonmetallic inclusion.And the specimen of pit defects on the inner surface also be tested in terms of metallurgical structure, scanning electron microscope and spectral analysis.As a result, it is identified that the pit defects were not metallurgical defects but cold-drawn defects after hot forming.The formation of pits were due to the pressure of iron oxide scale or other hard dirt on the surface of the steel pipe during the pulling process, or the peeling of the original raised skin on the inner surface of the steel pipe. The production process should strictly control the preparation work before cold drawing to prevent similar defects from recurring.