The invention discloses a hydropower generating device and a gas water heater. The hydropower generating device comprises a water-proof DC electric generator, an outer shell, an impeller gear, an impeller gear shaft and a supporting seat, wherein the impeller gear is installed on the impeller gear shaft which is arranged on the supporting seat; and a gear ring on an impeller gear body is meshed with a pinion at the front end of the water-proof DC electric generator at 90 degrees. The water-proof DC electric generator is driven to rotate by the rotation of the impeller gear body, thus outputting electrical energy. The hydropower generating device is adopted as a power supply device of the gas water heater, so that the traditional way of additionally adding a battery to the gas water heater whenin use can be thoroughly changed, and the electrical energy generated by the gas water heater can meet the need when the water is opened. The hydropower generating device and the gas water heater canbe integrated into a whole, and the battery is not needed, thus having the advantages of convenient use and no pollution for the environment.