The utility model discloses a demonstrator for simulating the ups and downs of a fish, comprising a transparent enclosed container used for containing liquid, wherein the transparent enclosed container is provided with a liquid injection opening which can be sealed, the upper panel of the transparent enclosed container is provided with a through hole, inside the through hole there is provided a press plate exerting a pressure on the liquid inside the transparent container, and a sealing film is connected between the press plate and the upper panel. The demonstrator for simulating the ups and downs of a fish realizes the ups and downs of a balloon through operating the press plate to change the size of the balloon, thereby avoiding balloon inflation and deflation operations in the experiment process, and really demonstrating the working principle of a fish maw.