A stable power-output device driven by waves includes buoys and a scaffold with the buoys, guiding tracks and rotating shafts installed. The scaffold floats on the waves, and wheels are installed at two sides of the buoys. Each two of the roller wheels are sandwiched between the related guiding track and the related side of each buoy, so that the roller wheels and the guiding tracks can work together. Guiding devices are installed on top of the buoys, and rotating shafts are connected to the guiding devices and float horizontally on the waves. Ratchets are installed on the short shafts in contact with the guiding devices. A shaft connector is adapted to connect each two of the rotating shafts together and form a long shaft.