In order to improve the precision of fiber-optic gyroscope, a novel fiber optic-gyroscope with double optical length was presented. By employing two polarization beam splitters, polarized light propagated twice along the slow and fast axes of the polarization maintaining fiber sensing coil in sequence to double its effective optical length and precision further. The thermal induced non-reciprocity in the proposed fiber-optic gyroscope was analyzed according to its special structure. With finite element analysis technique, modeling and simulation of the Shupe bias errors were also conducted. By modifying the 90°splicing point to the middle of the sensing coil or choosing appropriate fiber with special refractive index temperature coefficients , the Shupe bias error can be reduced significantly, which is low enough to meet the requirement of fiber optic-gyroscopes with high performance.