The normalized overlap factor between pump- and signal-light intensity profiles in the waveguide (Γ) is introduced, and be taken into account in the theoretic model of the Er-doped glass waveguide amplifier (EDWA). For an EDWA with buried graded refractive-index profile which is fabricated by Ag+-Na+ two-step ion-exchange on process Er-doped phosphate glass, the two dimensional refractive-index profiles, and the guide modes of both pump and signal light in the channel waveguide is obtained using the numerical methods. The normalized value of Γ with different fabrication parameters is calculated, and its effect on the gain of EDWA has been investigated. The results show that by tailoring the fabrication parameters, the overlap factor between pump and signal light in the waveguide can be optimized and the gain of EDWA can be enhanced. With Γ enhanced from 0.5 to 0.8, the gain of EDWA can be improved nearly 6 dB.