Er/Yb co-doped borate-silicate glass with 0.5mol% Er2O3 were fabricated by solid reaction. The concentration of Yb2O3 was in the range of 0.0mol% to 5.5mol%. The spectra of absorption, photoluminescence (PL) and up-conversion luminescence were analyzed. A critical value is found in the dependence of PL and absorption on the concentration of Yb2O3. When the concentration of Yb2O3 is greater than the critical value, the area of the absorption cross-section of Er3+ ions near 1535 nm has a linear increase with the increase of Yb2O3 concentration. Yb3+ co-doping makes the PL intensity to enhance and the PL spectra of Er3+ ions to broaden considerably. Quantitative analysis indicates that the inverse transfer energy from the acceptor to the donor does increase with the increase of Yb2O3 concentration. However, it could not be the dominaing factor causing the saturation of PL intensity. We ascribe the saturation of PL intensity to be due to the decrease of PL quantum efficiency and the saturation of excited states in the system caused by the increase of sensitizers. The sensitization of Yb3+ ions also results in the increase of Yb3+-2F5/2 and Er3+-4I11/2 populations, which promotes the cooperative up-conversion between Er3+ and Yb3+ ions.