A day/night switchable light adjusting device and light adjusting method thereof are provided. The day/night switchable light adjusting device is composed of a plurality of panels; each panel includes a reflecting surface and at least one lighting unit. Each lighting unit can emit various wavelength region lights, and the various wavelength lights are mixed on a light collecting component. A control unit is provided for adjusting the various wavelength region lights corresponding to day/night variation. A light intensity of a cyan region light or a blue region light is reduced for preventing an over-inhibition on a quantity of Melatonin.