We introduce FD-BPM to three-dimensional waveguides and calculate the refractive index of Er³⁺-Yb³⁺ co-doped phosphate glass waveguides.The simulation for a signal light (1.54 μm) and a pump light (0.98 μm) is performed in a Er³⁺-Yb³⁺ co-doped phosphate glass waveguide with FD-BPM.It shows that the scatter loss can be neglected and the optical field is symmetric in a buried waveguide.A buried Er³⁺-Yb³⁺ co-doped phosphate glass waveguide is demonostrated the best to make Er³⁺-Yb³⁺ co-doped phosphate glass waveguide lasers and amplifiers.