A gravity type rotary hinge water seal device of a radial gate is reliable in water stop, simple in structure, good in leakproofness and capable of conducting pressing with constant force. The gravity type rotary hinge water seal device comprises a fixed part, a movable unit and a counter weight. The fixed part comprises a fixed hinged support located between an upper steel plate and a vertical plate of a lintel embedded part of the radial gate, a rotary hinged shaft, a middle steel plate arc-shaped water stop base plate and a guide plate extending outwards from a lower steel plate. The movable unit is a water seal device assembly capable of rotating around the rotary hinged shaft. The movable unit comprises a movable hinged frame, a limiting idler wheel, a wheel shaft, a rotary hinge seal device and counter weight frames on the two sides of the movable hinged frame. The rotary hinge seal device further comprises a round head water seal device on the radial gate side, a P-shaped water seal device on the lintel side and long-tail flat and square head water seal devices on the left side and the right side. The counter weight comprises a counter weight iron block and a fastening screw, wherein the counter weight iron block is provided with a through hole.