The invention relates to a rotational-flow purifying device for a gaseous product in a fluidized-bed residual oil hydrogenation reactor. The device comprises the fluidized-bed residual oil hydrogenation reactor and a thermal high-pressure gas/hydrogen heat exchanger, wherein a thermal high-pressure flash tank is arranged between the reactor and the heat exchanger, and comprises a thermal high-pressure flash tank shell and an inlet distribution ring arranged in the shell; the inlet distribution ring has a spiral structure and defines a spiral channel together with the thermal high-pressure flash tank shell; a gaseous product inlet is formed in the thermal high-pressure flash tank shell; a tangential inlet guide pipe is mounted on the gaseous product inlet; the thermal high-pressure flash tank also comprises a micro-rotational-flow separator set arranged at the top in the thermal high-pressure flash tank shell. According to the device, the gaseous product in the fluidized-bed residual oil hydrogenation reactor is subjected to stepped rotational-flow separation, so that the entrainment quantity of liquid and catalysts in the gaseous product during a cold high-pressure separation process is reduced, the problem of blockage of the high-pressure heat exchanger is avoided, the energy consumption of a fluidized-bed hydrogenation device is greatly reduced, and the running cycle of the device is prolonged.