An underwater man-powered mechanical propeller includes a screw propeller arranged on a screw propeller shaft and encircled by a concentric protective ring. One side of the protective ring is provided with a supporting plate on which a first gear and a second gear mesh with each other, and a transitional gear and the second ear form a meshing pair. The drive shaft of the transitional gear passes through the supporting plate and has a second bevel gear, which meshes with a first bevel gear provided on the screw propeller shaft. The first gear is connected to a first connecting rod and the second gear is connected to a second connecting rod.