Abstract: Disclosed are an anti-backflow premixing device and a fuel gas water heater, where the anti-backflow premixing device includes a casing body, a premixing cavity is provided in the casing body, and the premixing cavity includes a gas outlet pathway; a baffle, the baffle is connected in the casing body, the baffle has a first position and a second position, the baffle can continuously move between the first position and the second position, the baffle obstructs the gas outlet pathway of the premixing cavity when at the first position, and the gas outlet pathway gradually opens when the baffle is moved from the first position to the second position; and a fuel gas passage, the fuel gas passage and the premixing cavity being in communication.