An optical measuring device includes a case, a reflective layer and a light collecting lens module. A measuring chamber and a channel, which is connected to the measuring chamber and is connected to an opening of the case, reside in the case. The reflective layer is disposed onto an inner surface of the measuring chamber. The light collecting lens module is located inside the channel. A light beam emits into the channel of the optical measuring device through an opening, passes through the light collecting lens module and enters the measuring chamber afterward.