A COB illuminating device includes a light source component, a control module, and a base plate component. The light source component includes multiple illuminating sets that are electrically coupled in series. The control module includes multiple control terminals, an input control terminal, an output control terminal and a constant current control unit. The multiple control terminals are respectively and electrically coupled to intersections between the multiple illuminating sets. The input control terminal is electrically coupled to a head illuminating set. The output control terminal is electrically coupled to a tail illuminating set. The constant current control unit is electrically coupled to the multiple control terminals, the input control terminal and the output control terminal. Besides, the constant current control unit activates a dynamic number of illuminating sets starting from the head illuminating set.