The invention discloses a bionic underwater flapping wing propelling device based on the manta ray. The device is composed of a servo motor, a reduction gear, a bracing wire wheel set, a fin ray mechanism, an outer framework and a skin. The fin ray mechanism is composed of multiple pressed rods and inhaul cables, the inhaul cables are connected with the bracing wire wheel set and the pressed rodsthrough V-shaped linear bearings, and therefore bracing wire wheels drive the pressed rods to move when rotating. The outer framework is installed on the vertical pressed rods of the fin ray mechanismand covered with the skin. The servo motor drives the bracing wire wheel set through a coupling and the reduction gear to rotate, the bracing wire wheel set pulls the fin ray mechanism to swing up and down, the outer framework on the fin ray mechanism and the skin covering the outer framework move and deform along with the swing of fin rays, and the flapping direction of flapping wings is controlled. The underwater flapping wing propelling device has the beneficial effects of being simple and reliable in structure, easy to control, capable of generating stable periodical propelling force, small in noise, high in stability and wide in application prospect.