The invention relates to the technical field of assay determination, and specifically relates to a measurement method for simultaneously measuring adsorbed-state and free-state methane in shale. According to the method, based on a low-field nuclear magnetic resonance technology, a nuclear magnetism T2 spectrum of methane in a shale sample is obtained, the marking conversion formula of nuclear magnetism signal amplitude and the quantity of a methane substance and the marking conversion formula of adsorbed-state methane and nuclear magnetism signal amplitude are obtained through calculation, low-field nuclear magnetic resonance is carried out on other shale samples, the marking conversion formulas of adsorbed-state and free-state methane are applied, and the quantities of the adsorbed-state and free-state methane in the adsorption/desorption process of the other shale samples are obtained. According to the measurement method, the quantities of adsorbed-state and free-state methane substances in shale at any time node can be dynamically detected in real time, and then the method is high in measurement speed and simple in operation and is extremely applicable for simultaneously measuring adsorbed-state and free-state methane in shale when the scales are the quantities of different states of methane in shale.