A light adjusting device for adjusting the luminance of a light source is provided. The light adjusting device comprises a rectifier module, a first switch element, a second switch element and a control module. The rectifier module receives an AC signal from a first power node and a second power node, and rectifies the AC signal to output a driving signal to the light source. The first switch element is coupled to the first power node. The second switch element is coupled to the second power node. The control module outputs a control signal to the first and second switch elements to control the conduction states of the first and second switch elements. When the first and second switch elements are turned on, the driving signal provided to the light source is interrupted.