A hydraulically balanced water-stop switch mechanism includes a water passage pipe, a water passage is formed between an inner pipe and an outer pipe of the water passage pipe. One end of the inner pipe is a closed end, and the other end is movably provided with a water-stop plug. A switch assembly is configured to cause movement of the water-stop plug. A chamber is formed between the water-stop plug and the closed end in the water passage pipe, and the chamber is provided with a ventilation hole. Preferably, the switch assembly includes a push rod, a automatic retractable and locking mechanism as applied in a ballpoint pen, and a elastic reset member for causing the water-stop plug to move toward the water outlet.