Impact localization can provide accurate location information for structural impact damages. As fiber Bragg grating has low sampling frequency and requires training samples, a method for structural impact localization based on optical fiber Sagnac sensing technique is proposed. The sensing system mainly consists of broadband light source, optical fiber Sagnac interferometers, photo-detectors and data acquisition card. The phase of the interferometer can be modulated when the sensing probe stuck on structure is affected by impact stress waves. Therefore, the output light intensity can be changed and converted to voltage signal by photo-detector. Firstly, the impact response signals are de-noised and direct current interferences are eliminated by wavelet transform. Secondly, the energy characteristics are extracted, time-domain signals are reconstructed by wavelet packet analysis, and the time of wavelet arrival to the two sensors is acquired. Lastly, impact position is calculated by time-difference method. The result shows that 35 impacts on the steel pipe with the length of 100 cm have a maximum error of 0.65 cm, and a maximum root-mean-square error of 0.36 cm, respectively. This research can provide a novel and available method for structural impact localization.