An illumination adjusting device for receiving power from a power supply and supplying output voltage to a lighting unit is provided herein. The illumination adjusting device includes a detector for detecting ambient illumination intensity and generating an ambient illumination value according to the ambient illumination intensity; a comparator for comparing the ambient illumination value with a predetermined value, the comparator outputting a first power signal when the ambient illumination value is lower than a predetermined value; an adjustor for adjusting the output voltage according to the first power signal, as a result, light emitted by the lighting unit is controlled within a predetermined illumination intensity range. A related illumination adjusting method is also provided.