Objective To study the online automatic detection system for geometric size of cartridge inner hole. Methods Firstly, on the basis of the design requirement and processing technical characteristics of the geometric size of cartridge inner hole, the gas electric measurement system was used for measurement. According to the measuring characteristics of gas electric measurement system, the mechanical movement structure was designed. Secondly, the automatic detection system was designed to realize automatic loading and unloading, online measurement, data acquisition and processing and touch screen monitoring. PLC was used as the core of the automatic control, data acquisition and data processing. Based on the characteristics of measurement data, a highly efficient communication data processing method was put forward to solve the problems of serial communication data format dispersion, operation difficulty and data mismatching. Finally, the standard ring gauge was used to study the multiplicity and the linearity of the automatic detection system, and accurate measurement results were displayed on the touch screen. Results Research showed that the maximal deviation of the system was measured to be 4 μ m, the discrete degree of measurement data was low, and the repeatability precision of the system was high. As seen from the fitting straight line, the linear characteristics of the system was good. Conclusion Through the experimental study of the system, it was ensured that the detection efficiency of cartridge was 800 pieces per hour with high detection accuracy and reliability.