The present invention relates to an adsorbed gas content measuring instrument and its testing method. The instrument comprises sealed cylinders, gas collecting graduated canisters and a test box, wherein the sealed cylinders are provided with valves, the gas collecting graduated canisters are equipped with drain hole adjustment valves and vent hole switching valves fit for the valve ports, and there is a heating element and a temperature controller in the test box. The method is mainly as follows: put gas-contained samples and saturated brine into the sealed cylinder, seal the cylinder, feed water in the test box and heat the box, place the sealed cylinder into the water bath of the test box, introduce saturated brine into the gas collecting graduated canister, connect the canister and cylinder, open the drain hole of the gas collecting graduated canister, record the liquid level of the graduated canister and cut off the connection after the test. The present invention has a compact structure, small volume, good gas tightness, accurate measurement, uniform heating and simple testing operation, is easy to move and carry, and suitable for field application. It can be specially used for collecting and measuring various gases, especially small-volume gas, thus it is more practical in applications.