The invention discloses a water drain pipe capable of preventing mosquitoes and flies, and belongs to the field of water supply and drain articles. The water drain pipe which can drain water easily and smoothly and prevent injurious insects from entering effectively comprises a square pipe-shaped water drain pipe, the outer end head of the water drain pipe is sleeved with a sleeve which is in the same shape with the outer end head of the water drain pipe but bigger in size and can be plugged by the outer end head, the front pipe wall and the back pipe wall of the sleeve are cut from the middle of the inner end head to the outward, upward and downward into an upper sleeve and a lower sleeve, the upper sleeve and the lower sleeve are movably connected to the outer wall faces of the upper pipe wall and the lower pipe wall adjacent to the outer end head of the water drain pipe through hinges respectively, a set of teeth which can be meshed with each other are processed at the positions, where cutting is started, of the front pipe wall and the back pipe wall of the upper sleeve and the lower sleeve, and the centers of the teeth and the axes centers of the hinges are arranged concentrically; the lower pipe wall of the lower sleeve is provided with a hole or a leaking opening, the leaking opening faces downwards and is connected with a water containing cavity which contains a sponge and is processed with a plurality of leaking holes, an arc-shaped sliding channel is processed on the lower portions of the teeth of the lower sleeve, and a rivet-shaped blocking rod is fixedly connected with the wall of the water drain pipe through the sliding channel.