Testing the overlength pile in urban subway engineering is the key for quality control. Because of the limitations of urban construction sites, it is difficult to achieve efficient results with the normal testing method. Optical fiber measuring technique has gradually replaced the application of the traditional strainometer in geotechnical engineering testing as a result of the unique advantages it offers. In this study, first, the principles underlying the optical fiber measuring technique are introduced and its application in pile foundation testing in subway engineering is discussed. Then, combined with engineering practice, the optical fiber measuring technique is employed to study the law of development of strain in the overlength pile with the loading process under the action of vertical load. Finally, some meaningful conclusions are arrived at and reference foundations are provided for the optimized design of the same kind of engineering.