Erbium-ytterbium co-doped phosphate glass planar waveguides have unique advantages in heat dissipation and nonlinear effect suppression. It can be developed as the gain medium of near-infrared 1.5 μm high average power solid lasers, which is of great significance. Erbium-ytterbium co-doped phosphate glass planar waveguides are fabricated by optical contacting and thermal bonding. The effect of pre-bonding step-temperature on bonding quality is studied. At the same time, the influence of bonding temperature and bonding time on the thickness of molecular diffusion layer at bonding interface is obtained by electron probe surface analysis (EPMA). According to Fick's second law, the diffusion mechanism of Yb³⁺ in core glass under one-dimensional equivalence hypothesis is discussed, and the molecular diffusion model of the solid-solid interface during bonding process is established. Finally, a high-quality sandwich structure planar waveguide with 100 μm core layer and bonding strength of 11.63 MPa is obtained by selecting optimal heat treatment temperature curve.