Abstract: A helmet having a warning device includes at least one illuminator externally mounted on a helmet body. A solar power supplying device has a solar panel fixedly mounted on a top surface of the helmet body for absorbing solar energy, which can be converted into electrical energy by means of a control circuit board and saved to an accumulator. The accumulator supplies the illuminator with required electricity to generate illumination. The control circuit board can detect if there is any solar energy gathered by the solar panel so as to automatically control the illuminator to generate illumination or not. A switch, when the illuminator automatically generates illumination, can be activated again to forcibly control the illuminator to continually generate illumination or not.