In recent decades,the FBG (Fiber Bragg Grating) sensors are widely used in SHM (Structural Health Monitoring) fields for the characteristics of its simple structure,tiny volume,light weight,anti-corrosion,and anti-electromagnetic interference.However,there are two defects that limit its application in some fields.Firstly,the low sampling frequency due to the induction principle provides a big challenge to the solver algorithm.Secondly,real-time online monitoring which requires quick and efficient signal processing for the long time series is the development direction of SHM and also needs progress in the fast solver algorithm.For these two reasons,we propose a novel method based on the PSD (Power Spectrum Density) to analyze the sampled data from FBG sensors in this paper and it is applied in an online crack initiation monitoring system and obtain some satisfactory results.A comparison with the preliminary research indicates the accuracy and rapidity of the proposed algorithm and points out the applicability in the real-time online monitoring system for crack initiation.