A real-time wavelength correction system for visible light is co-operated with an optical system to make a parallel light beam split into a zero-order diffractive parallel light beam and a first-order diffractive parallel light beam. The zero-order diffractive parallel light beam focuses on a first back focal plane to form a first light spot. A drift of the first light spot is applied to determine an angular drift of the parallel light beam. The first-order diffractive parallel light beam is focused on a second back focal plane to form a second light spot. A drift of the second light spot is applied to determine an angular drift of the first-order diffractive parallel light beam. The angular drifts of the parallel light beam and the first-order diffractive parallel light beam, which are changed with real time temperature variation, are applied to correct the wavelength of the parallel light beam.