The characteristics of an Er3+/Yb3+co-doped phosphate glass waveguide laser are investigated on the basis of the working theory of a waveguide laser. Using the overlapping factor, the ratepropagation equations of the fourlevel system of the phosphate glass waveguide laser are simplified. Without considering amplified spontaneous emission, the relations between the laser outputs and the laser parameters are researched by numerical simulations and the relation curves of Er3+/Yb3+ ion concentration, pump powers and waveguide lengths with the laser powers are obtained. The result shows that selecting right Er3+/Yb3+ ion concentration is the key for the optical waveguide laser. The best fit results can be obtained from the following parameters in Er3+ ion concentration of 2.0×1026/m3, Yb3+/Er3+ concentration ratio of 7-10, and a waveguide length about 20 cm, when the pumping power is 80 mW at a wavelength of 980 nm.