An energy-saving lamp includes a light emitting module, a light sensing module and a control module. The light emitting module includes a plurality of light emitting diodes for emitting light. The light sensing module detects a light intensity of the light emitting module and generates a detection signal according to the light intensity of the light emitting module. The control module receives the detection signal generated by the light sensing module and adjusts the total number of lit up light emitting diodes of the light emitting module according to the detection signal.