The cantilever plate structure is widely used in wind tunnel nozzle segments. During the operation of the wind tunnel, the flexible steel plate will bear various complex loads such as shearing, bending and twisting, etc. The bolts of the plate are prone to loosen due to the fatigue damage. The methods based on the electromechanical impedance, sub-harmonic resonance and vision have been presented in the literature. However, the mentioned methods cannot detect the bolt looseness effectively in a harsh environment. To address this issue, we presented a novel method to detect the bolt looseness for the cascaded structures based on the operational strain estimated from Fiber Bragg Grating (FBG). In this work, an improved method of the operational modal analysis (OMA) presented in our previous work is employed. And then an enhanced Structural Damage Indicator (SDI) based on the operational strain is presented to detect and locate the loose bolts. Both the results from experiments and simulations showed that the SDI can detect and locate the loose bolts accurately.