Abstract: A warning device disposed on a top of an umbrella comprises a base, an LED unit and a transparent cap. The base is formed in a lower end thereof with an accommodation space flexibly expanding downwards, on an upper end of the base is formed an outer threaded portion, and in a top surface of the upper end of the base is formed a concave space for placement of the LED unit. The transparent cap is formed in a lower end thereof with an inner threaded hole to be screwed with the outer threaded portion of the base, and a concave portion is formed upwards from the inner threaded hole. The transparent cap is further formed with plural protrusions on an upper end thereof.