The invention discloses a rapid underwater algae removal mechanical device. The mechanical device comprises a duckweed removal device body A and an algae removal device body B, an opening is formed inthe center of the bottom of the duckweed removal device body A, the duckweed removal device body A sleeves the algae removal device body B through threads, the duckweed removing device body A comprises a duckweed removal outer barrel (17), a top cover (15) covers a top opening of the duckweed removal outer barrel (17) in a sealed mode, a cylindrical check ring (16) with the depth smaller than thedepth of the duckweed removal outer barrel (17) is fixedly arranged on the inner bottom surface of the top cover (15), blades (14) are arranged in the cylindrical check ring (16), the algae removal device body B comprises an algae removal outer barrel (7), a coarse filter screen (3) is arranged at a water inlet in the side wall of the top edge of the algae removal outer barrel (7), an algae removal device top cover (2) covers a top opening of the algae removal outer barrel (7) in a sealed mode, and a filter barrel (4) of a stainless steel filter screen is concentrically arranged in the algaeremoval outer barrel (7). The mechanical device has the advantages that the algae removal and duckweed removal functions can be achieved, the use is simple, the algae removal efficiency can be improved, the energy source for extracting algae liquid can be saved, and the automatic cleaning function can be achieved.