In order to solve the problems of low precision, poor reduction degree of internal surface morphology and inconvenient measurement of gun barrel inner diameter, an automatic measurement system of gun barrel based on laser triangulation is designed. The system mainly uses laser sensor, single chip microcomputer and stepping motor to design the mechanical measurement structure of propulsion type, and uses the principle of geometric centering to measure the whole body of gun automatically; The minimum measuring inner diameter is 60mm; The sampling data is processed by filtering and cubic spline interpolation, which greatly reduces the measurement error. The measurement system is used to measure the body pipe of different caliber. The measurement results show that the measurement system can work stably and can measure the body pipe with different inner diameter. The measurement resolution is 0.001mm, the measurement accuracy is 0.02mm, the measuring point is accurate, the functions of each part of the upper computer are normal, and the wear defects of the inner wall of the body pipe can be restored well. The measurement system has the advantages of small measurement error, high efficiency, simple operation, high automation and can also be used in the same kind of pipe measurement.