The invention discloses an underwater multistage natural gas gas-liquid separation device and method. The device comprises a first-stage separation device, a second-stage separation device and a third-stage separation device which are sequentially connected in series from bottom to top. The first-stage separation device comprises a horizontally arranged tank, a gas inflow baffle arranged near a gas inlet of the tank, an agglomeration screen mesh arranged behind the gas inflow baffle, and a volute separation unit arranged on a gas outlet of the tank. The second-stage separation device comprises a cyclone core pipe connected with the gas outlet of the tank, multiple cyclone mechanisms arranged in the cyclone core pipe, and a first liquid drainage sleeve arranged outside the cyclone core pipe in a sleeving mode. First liquid drainage side seams are formed in the positions, on the rear sides of the cyclone mechanisms, of the lower pipe wall of the cyclone core pipe. The third-stage separation device comprises an agglomeration core pipe connected with an outlet of the cyclone core pipe, a mist capturing screen mesh arranged in the agglomeration core pipe, and a second liquid drainage sleeve arranged outside the agglomeration core pipe in a sleeving mode. Multiple second liquid drainage side seams are also formed in the lower pipe wall of the agglomeration core pipe.