The invention discloses an assembly type buoy-powered mast-combined crane device. The device comprises a float system, a hoisting system and a mast amplitude variation system, wherein a supporting platform is anchored on the float system, and the hoisting system and the mast amplitude variation system are arranged on the supporting platform; the mast amplitude variation system comprises an amplitude variation winch, first masts, second masts, third masts and an amplitude variation steel wire rope, the first masts, the second masts and the third masts are sequentially hinged, a steel box is fixedly connected between upper ends of the two third masts, and the amplitude variation winch is connected with the upper ends of the third masts through the amplitude variation steel wire rope; the hoisting system comprises a hoisting winch, a hoisting steel wire rope and a hook, and the hoisting winch controls the hoisting steel wire rope to pull a heavy object. The whole device is supported by the float system, a proper hoisting point can be found through the mast amplitude variation system, and finally, the heavy object is hoisted by the hoisting system; the device has the advantages that the device is simple to assemble and fast and convenient to use, and the construction cost can be saved.