The residual thermal stress distribution is evaluated in the silicon-based silica optical waveguide with finite element method and the stress birefringence is obtained. The influence of stress on polarization is analyzed. It is concluded that birefringence is induced mainly by the different expansion coefficient between the substrate and glass layers. The birefringence-free planar optical waveguide can be obtained by increasing the level of dopants to match the thermal expansion coefficient of glass layers to that of the substrate.