An illuminance calibrating device includes a reference light source and an illuminance detection device. The reference light source, which generates light with a specific wavelength and a source illuminance, adjusts the source illuminance to be a first illuminance according to a control signal. The illuminance detection device includes a shading plate, an illuminance detector, and a controller. The shading plate is configured to lower the first illuminance to a first shading illuminance. The illuminance detector detects the first shading illuminance to generate a detection signal. The controller generates the control signal and calculates a ratio of the first illuminance to the first shading illuminance according to the detection signal.