Waterproof electrical connecting device

A waterproof electrical connection device includes a waterproof casing, a socket assembly, a wire element, and a waterproof plug. The waterproof casing includes a wiring port. An inner surface of the waterproof casing includes an annular step surface facing the wiring port. The wiring port is covered by an end cover. The socket assembly is placed inside the waterproof casing. The waterproof plug is arranged in the wiring port to seal it. The wire element is connected to a connector of the socket assembly and passes through the waterproof casing via the waterproof plug. The waterproof plug has an inner end and an outer end opposite to the inner end. The inner end is arranged toward the socket assembly and includes a longitudinal ring rib. The end cover is fastened to the waterproof casing to press the outer end, so that the longitudinal ring rib presses the annular step surface.