The invention provides a coal mine underground coal seam gas content automatic measuring device which comprises a first mounting shell, a second mounting shell, a coal sample tank, a tank cover, a supportable lifting seat structure, an anti-explosion high-speed motor, a rotary crushing frame structure, an anti-skid pad, an auxiliary gathering crushing seat structure, a first conveying pipe, a first electromagnetic valve, a gas-solid separator and a second conveying pipe. The device comprises a methane concentration sensor, a second electromagnetic valve, an intelligent flowmeter, a first gas pressure sensor, a third conveying pipe, a fourth conveying pipe, a third electromagnetic valve, an anti-explosion vacuum pump, a measuring cylinder, a liquid pressure sensor, a fourth electromagnetic valve, a rubber hose, a storage tank, a rodless cylinder, a second gas pressure sensor, a temperature sensor, a PLC, a control panel and an operation switch. The device has the beneficial effects that through the arrangement of the supportable lifting seat structure, the rotary crushing frame structure can conveniently ascend from the interior of the coal sample tank, and large samples can be conveniently placed and measured.