Abstract: The researches on the measurement, performance improvement and the mechanism of photo-darkening both in china and abroad are reported in this paper. The photo-darkening effect on the absorption spectra, excess losses at a special wavelength and the fluorescence output power is showed. A new photo-darkening resistance fiber is manufactured. The performance of the photo-darkening resistance improved as a factor of 15, compared with the normal Yb doped silica fiber, and the slope efficiency is 81.8%. An 800 Wfiber laser prepared by the new fiber is realized, and the power decay is not observed.