The invention provides a variable-pressure gravity discharging method for oil slag in a hydrogenation reaction product of a suspended bed. The method comprises the following steps: firstly carrying out primary gas-liquid separation on the hydrogenation reaction product of the suspended bed in a thermal high-pressure separator, and enabling the separated high-pressure oil slag to flow into a lock hopper through gravity, wherein the lock hopper is pressurized to be balanced with the pressure of the thermal high-pressure separator by virtue of thermal high polymer gas before the high-pressure oil slag flows into the lock hopper; and depressurizing the high-pressure oil slag to a pressure balanced with the pressure of a thermal low-pressure separator, enabling the depressurized oil slag to flow into thermal low-pressure separator from the bottom of the lock hopper through gravity, and carrying out secondary gas-liquid separation, and discharging the separated product into subsequent procedures. The invention further provides a discharging device utilizing the variable-pressure gravity discharging method for the oil slag in the hydrogenation reaction product of the suspended bed. The method and the device are used for treating the oil slag in the hydrogenation reaction product of the suspended bed, and the high pressure drop of the oil slag during the flow in a pipeline can be basically eliminated, so that the serious wear of the oil slag pipeline and an assembly of the oil slag pipeline is avoided, and the service life of the device is prolonged.