The invention discloses an online discharging device for residues in a supercritical water gasification furnace and an online discharging method. The online discharging device comprises a supercritical water gasification furnace, a cooler, a residue discharging lock hopper, a high-pressure water pump, a residue storage tank, a first valve, a second valve, a third valve, a fourth valve and a fifth valve, wherein an outlet of the supercritical water gasification furnace is communicated with a residue inlet in a side wall of the residue discharging lock hopper through the cooler and the third valve; a water outlet at the bottom of the residue storage tank is divided into two paths; one path is communicated with the water inlet in the side wall of the residue discharging lock hopper through the fifth valve; the other path is communicated with the water inlet at the top of the residue discharging lock hopper through the first valve; the water outlet at the top of the residue discharging lock hopper is communicated with the water inlet of the residue storage tank through the second valve; a residue outlet at the bottom of the residue discharging lock hopper is communicated with the residue inlet at the top of the residue storage tank through the fourth valve; the top of the residue discharging lock hopper is communicated with a second pressure gage for measuring inner pressure of the residue discharging lock hopper. The device and the method can be used for realizing smooth residue discharging of the supercritical water gasification furnace.