Gain and quantum conversion efficiency(QCE) of Er3+-Yb3+ codoped phosphate glass waveguide amplifier at large signal operation are studied. From the definition of quantum conversion efficiency the analytical expression relating gain, pump power and quantum conversion efficiency is obtained. By solving the rate and optical power propagation equations describing the Er3+-Yb3+codoped system, the effects of Er3+-concentration, Yb3+-concentration, the ratio of Yb3+-to Er3+-concentration, pump power and amplifier length on quantum conversion efficiency are discussed. It is shown that increase of Er3+-concentration and of amplifier length can improve quantum conversion efficiency, in order to reduce up-conversion caused by high Er3+-concentration, high Er3+-concentration must match corresponding high Yb3+-concentration, improvement of Yb3+-concentration reduces quantum conversion efficiency, the rate of 1～2 of Er3+-Yb3+ codoped concentration is available.