Shupe error is one of the major technical bottlenecks affecting the application of high-precision fiber-optic gyroscopes (FOGs). To address the problem that the temperature performance of high-precision FOGs deteriorates after the assembly and packaging of the optical path, this paper analyzed the temperature error of the FOG optical path and revealed the effect of the assembly stress of the interference optical path on Shupe error. Temperature errors caused by the assembly stress under different conditions were tested. Three initially installed FOGs with a full-temperature zero-bias range of 0.11 (°)/h were chosen and the tail fibers of their fiber coils and Y-waveguides were bonded in different fashions for different assembly stress symmetry. When the bonding length of the tail fibers was 30 cm, the zero-bias ranges of the two FOGs with poor assembly stress symmetry became 0.24 (°)/h and 0.43 (°)/h, signifying significant deterioration of FOG temperature performance. In contrast, the one with better assembly stress symmetry became 0.13 (°)/h, indicating no significant deterioration of FOG temperature performance. The results show that poor assembly stress symmetry of the interference optical path could lead to distinctive deterioration of FOG temperature performance. In addition, the deterioration degree is directly related to the symmetry of assembly stress. More precisely, better assembly stress symmetry results in smaller deterioration.