The invention relates to mosquito preventing gate applied to bilge wells and rainwater wells, and belongs to the field of devices, equipment or systems for preventing mosquitoes in manholes. An intra-well environment is isolated from an external environment in a mode of adding a drain cabin at the lower side of a well cover main body; when drainage is required in the case of such situations as rain and the like, surface runoff enters a diversion cabin and the drain cabin, wherein the diversion cabin is in charge of guiding some sewage into a water storage bucket by virtue of a diversion pipe,and the weight of the water storage bucket is increased and a gear rack is pulled downwards, so that a turning plate is opened by virtue of a delayed turning mechanism, so that drainage is achieved; and when the surface runoff is reduced, the weight of the water storage bucket is gradually reduced and the turning plate is gradually reset, so that sealing is completed once again.