A waterproof fastening assembly secures a first structure to a second structure. The first structure defines a number of through holes and a number of flanges surrounding the through holes, respectively. The second structure defines a number of screw holes and includes a number of LED light source modules. A connection shank extends through a corresponding through hole and screws in a corresponding screw hole. An elastic waterproof ring comprises an annular body and a number of protrusions protruding from a top surface of the body. When the first structure is secured to the second structure, the connection shank presses the protrusions to enable the elastic waterproof ring to deform outwardly and inwardly simultaneously. The flange of the first structure resists the outward deformation of the elastic waterproof ring, meanwhile the body of the elastic waterproof ring inwardly engages the connection shank.