The invention discloses an automatic measurement device and measurement method of underground coal mine seam gas content. A PLC controller controls the on-off and start-stop of each of an electromagnetic valve I, an electromagnetic valve II, an electromagnetic valve III, an electromagnetic valve IV, an electromagnetic valve V, an anti-explosive servo motor I, an anti-explosive servo motor II, andan anti-explosive high-speed motor in a control system; a central multifunctional processing display is used for estimating the lost gas quantity by sensing the measurement data of a liquid pressure sensor I, a liquid pressure sensor II, a gas pressure sensor, a temperature sensor, a methane concentration sensor, and an intelligent flowmeter; therewith, a coal sample is smashed through a preset program without opening a coal sample tank, and the gas dissipation is reduced; and finally, the system automatically corrects the gas content at each stage, and computes the final coal seam gas contentand issues a report. The one-key operation can be realized through a touch screen, the gas desorption quantity can be automatically metered, the measurement efficiency of the coal seam gas content isgreatly improved, and the determination result is accurate, the structure is simple, and the process operation is convenient.