The pulsed neutron source logging technology was applied to the detect underground coal seam type and formation composition. The structure of the logging tool was designed according to the pulsed neutron source technology. The neutron emission was controlled by the detector control device to convert it into an electric pulse signal and the pulse sequence which was controlled to complete the detection of the gamma ray in the neutron reaction process, with energy spectrum and time spectral parameters. Through the experimental analysis of the actual mine, the results showed that this pulsed neutron detector could accurately measure the underground coal gas composition.