A medication concentration detecting device includes a medicine container, a three-way pipe, a light emitting member, a first light receiver and a processor. The medicine container has a chamber configured for accommodating nebulized medicine. The three-way pipe has a passageway connected to the chamber for the nebulized medicine to flow along the passageway. The light emitting member is disposed on the three-way pipe and configured for emitting a light beam toward the passageway. The first light receiver is disposed on the three-way pipe and configured for receiving the light beam and outputting a luminous flux signal. The processor is connected to the first light receiver and configured for calculating a luminous flux reference value according to the luminous flux signal. The luminous flux reference value is used for determining whether outputs a low nebulized medicine concentration warning.