While the strain modal index method is commonly used in damage detection, current damage indices based on strain often need complete measured modes and modal parameters before and after damage, both of which are difficult to obtain in practical engineering. We combined the fiber Bragg grating (FBG) sensor and inverse finite element method (iFEM) in order to reconstruct the global strain field based on measured strain data of limited points. We acquired an approximate complete measured mode based on the global strain field. Thus, the strain modal damage index has a natural advantage in directly making use of strain. Then, we proposed a direct damage detection index based on strain model shape, which could locate damage with only the strain modal data after injury and did not require data before damage. The calculation results showed that the global strain field could be reconstructed quickly based on FBG and iFEM, thus supporting damage detection based on the strain modal method. In the strain modal difference curve, the positive and negative changes occurred at damage locations besides the constrained point, and the mutation degree was slightly different than damage degree, but the pattern was similar. In addition, the proposed direct damage detection index had nothing to do with the model order after regularization. The results of the actual plate experiment also validated the proposed method.