The characteristics of Er3+-doped phosphate glass waveguide amplifier (EDWA) are investigated. Using the overlapping factor, the rate-propagation equations of the four-level system of EDWA are simplified. With taking account of up-conversion and amplified spontaneous emission (ASE), the relation curves are obtained by numerical simulation between the gain and the parameters such as Er3+ ions concentration, pump power and waveguide length, and the theoretic results are consistent with the experimental results. Selecting suitable erbium ions concentration is the key of fabricating EDWA. Moreover, in order to improve the EDWA performance, the parameters are optimized such as pump power and waveguide length.