the balance measuring cylinder 4 since the liquid storage tank 5 is fixed the liquid level in the balance measuring cylinder 4 gradually dropping with the ascending of the gas collection measuring cylinder 3 and the balance measuring cylinder 4 when detecting that the difference of the liquid level in the gas collection measuring cylinder 3 and the liquid level in the balance measuring cylinder 4 is less than a set accuracy the upper computer 7 controlling the driving device 25 of the lifting device 12 to stop working the gas pressure in the gas collection measuring cylinder 3 being equal to the external atmospheric pressure again when the liquid level in the gas collection measuring cylinder 3 reaches a preset lower limit the normally opened electromagnetic valve 20 being closed when the upper computer 7 controls the driving device 25 of the lifting device 12 stops working namely the gas pressure in the gas collection measuring cylinder 3 being equal to the external atmospheric pressure the corresponding measuring cylinder volume difference relative to an original liquid level namely the change value of the liquid level upper limit value of the liquid level of the gas collection measuring cylinder 3 at this moment being the gas output of the reservoir sample the upper computer 7 finishing once gas content measurement according to the measured and recorded change value of the liquid level in the gas collection measuring.