The present invention provides a melatonin suppression extent measuring device, which is able to receive and process a light signal to a spectral data, process the spectral data to a percent value of melatonin suppression based on a reference wavelength parameter and a reference quanta parameter through a conversion module, and display the percent value of melatonin suppression; therefore, users can exactly know how many melatonin suppression extent are caused by the light environment which is exposing themselves body.