The gain characteristics of Er3+/Yb3+-codoped phosphate fiber amplifier pumped with light of 980nm wavelength are theoretically studied in this paper. With the help of rate equations and the propagation equation of Er3+/Yb3+-codoped phosphate fiber, we analyzed the effects of concentration of erbium and ytterbium ions, pump power, signal power and fiber length on the gain characteristics of the co-doped fiber amplifier. Comparison with singly erbium-doped fiber amplifier was also made. It is shown that the sensitization of Yb3+ decreases the clustering effect of erbium ions in the fiber and the gain and pump efficiency of Er3+/Yb3+ co-doped fiber are apparently higher than that of singly erbium-doped fiber. Numerical results also demonstrate that considerable signal gain of 10dB can be achieved in a 3.2cm-long fiber amplifier with 20dBm(100mW) pumping power at 980 nm.