To solve the performance labefaction problem of fiber-optic gyroscope (FOG) when its temperature changed, modes coupling caused by thermally-induced refractive index of highly birefringent optical fiber was analyzed to be the main reason of limitting the precision of gyro in theory. Finite element method was used to calculate stress distributing. Based on the stress state of high or low temperature, the variational instance of birefringence was conjectured respectively. Six typical temperature points were selected as work condition to calculate thermal stress. The result shows that thermal-induced birefringence variations reduces accordingly based on different temperature, tallying the experimentation well from the extinction ratio. The result indicates double-coated optical fiber loop changes its birefringence at temperature range , for the expanding coefficient of fiber dismatching that of the framework and glue. Thermally-induced birefringence is 1×10-4 at 60��, the same order with its own birefringence of 5.5×10-4, and it deteriorates the polarization in optical fiber and decrease gyro precision furthermore.