In order to detect tunnel deformation more sensitively, it is necessary to pre-stretch the strain optical fiber between anchor points when using fixed-point method to fix the sensing optical fiber. A certain amount of pre-tensile strain is thus generated inside the optical fiber due to the pre-stretching. Previous test results indicate that have shown that the monitoring results of the fiber will be affected by different pretension degrees. In order to reduce the monitoring error of optical fiber and improve the accuracy of monitoring, a scheme of optical fiber tension test was designed according to the strain characteristics and mechanical properties of sensing optical fiber, and a test system was then constructed to carry out the optical fiber tension test under different initial pre-tension values. By calculating and analyzing the experimental data, the optimal initial pre-tension values of two special sensing fibers were obtained. Combined with the above research results, the sensing optical fiber was applied in the Dahongmen Bridge Station–Heyi Station in the third phase of Beijing Metro Line No. 8. The results of this study can provide reference for the layout of sensing optical fiber in structural health monitoring engineering.