Silica glasses containing different contents of Al2O3, Yb2O3 and P2O5 were fabricated by Sol-Gel method combined with high temperature vacuum sintering. Changes in UV transmission, fluorescence spectra under the UV excitation, and X-ray photoelectron spectra(XPS) of Yb4 d caused by P~(5+) and Al~(3+) ions co-doping in Yb~(3+)-doped silica glasses were comparatively investigated. The related mechanisms were discussed. Results show that the strong absorption bands in the range of 190 nm to 300 nm in Al~(3+)/Y...