The invention relates to a device and a method for correcting calculated free amount of coal seam gas and measuring residual adsorption, and belongs to the technical field of coal mine safety. The device comprises an adsorption tank body, a gas pressure reducer, a roots vacuum pump, a gas pressure regulating valve and a gas storage tank, wherein the adsorption tank body allows coal sample storageand is arranged in a constant-temperature air bath environment, and a rotating platform is mounted below the adsorption tank body; an X ray source and a ray detector which right face the adsorption tank body are fixedly mounted in constant-temperature air bath, the ray detector is connected to a data acquisition and analysis system, the gas storage tank is connected to the gas pressure regulatingvalve, the other end of the gas pressure regulating valve is connected to a first port of a three-way valve, and a second port of the three-way valve is connected to a gas source through a gas needlevalve I and the gas pressure reducer; a third port of the three-way valve is connected to the roots vacuum pump through a gas needle valve II. Through the adoption of the device and the method, porosity and free gas content of a coal sample under different gas pressure and different loaded conditions are measured precisely.