A method for heating oil shale underground in situ. Shale oil and fuel gas can be obtained from an underground oil shale seam in situ, and the fuel gas can also be obtained from an underground coal seam in situ. Wells are drilled downwardly reaching an operation region of an underground oil shale ore bed. Electricity for partial discharge of the ore bed is conducted into electrodes, and a plasma channel is formed in the ore bed and subjected to breakdown by the electricity; after the resistance of each of two electrode regions is lowered, the two electrodes are used for conducting currents into the plasma channel in the oil shale ore bed; the oil shale ore bed is heated under the resistance heating function of the plasma channel; and released heat is used for realizing thermal cracking and gasification of fixed organic carbon in the oil shale ore bed.