An illuminating device and a luminance switching device thereof for controlling luminance of light emitting states of LED light source are provided. The luminance switching device includes a changeover switch which is electrically connected to the luminance switching device for switching the light-emitting states of the LED light source and has switching modes corresponding to the light-emitting states, a detecting circuit for detecting the switching mode corresponding to the changeover switch, a memory unit for storing luminance values of the LED light source corresponding to the switching modes detected by the detecting circuit, converting the luminance values of the LED light source into a luminance control signal, and transmitting the luminance control signal to the detecting circuit, and a power conversion circuit for receiving the luminance control signal transmitted from the detecting circuit, such that the LED light source produces luminance of the light-emitting states corresponding to the switching modes.