A pen instrument is provided with a controllable whistle. The whistle includes a tubular body; a sleeve connected at a rear end to a first open end of the tubular body and at a front end to a rear end of the pen instrument to connect the whistle to the pen instrument; a whistle-producing reed structure fixedly provided in an inner space of the tubular body and including a reed; and a cut-off device provided at a predetermined position on the tubular body and operable by a user to allow or cut off flowing of air through the inner space of the tubular body. A balloon is connected to a second open end of the tubular body and inflated when the cut-off device is in a position allowing air to flow through the tubular body into the balloon.