The spectral shape multiplexing technology and the simulated annealing algorithm are used to theoretically analyze and experimentally study on the spectral overlap of fiber Bragg gratings(FBGs).When the spectra of FBGs are fully or partially overlapped,the recognition of 2,3 and 4 gratings multiplexing is discussed.In this research,no more than 4 spectra overlapped gratings are well distinguished and their recognition accuracies are within 10 pm,which are in agreement with the theory.The method provides an alternative solution for the multiplexing demodulation of fiber grating sensing network in large scale.