An inlet valve for water sealing in a downstream direction comprises a valve body, a water inlet valve core assembly, a buoy, a lever transmission mechanism, a water outlet movable core and a valve body top cover. The water inlet valve core assembly comprises a seal ring pressure cover, a water inlet seal ring, a movable valve core, a valve cover and a core body. A water guiding head of the movable valve core penetrates a water guide hole to extend out; the water inlet seal ring is fixedly connected with a cover body part, a water outlet base in a valve core cavity extends into the water guide hole in the seal ring pressure cover, the water guide head of the movable valve core is slidably matched with a water hole in the water outlet base, and the water inlet seal ring adheres to the end of the water outlet base to achieve liquid seal. The core body is fixed to the lever transmission mechanism, and the core body is matched with a core base part. When the buoy is at the upper position, the water inlet seal ring leaves from the water outlet base, water comes out of a water outlet pipe, and meanwhile, water also enters a water sealing cavity; when pressure of the water sealing cavity is larger than that of water in a water inlet pipe, the water inlet seal ring adheres to the water outlet base, and water inflow is stopped. The pressure direction of the water sealing cavity is the same as the water outlet direction of the water outlet base, and the sealing performance is improved.