Based on the index modulation method, an Interleaved Sampled Fiber Bragg Grating (ISFBG) is designed by changing the index modulations of the adjacent sub-gratings in each seeding grating. And the characteristic of reflecting spectrum of ISFBG is analyzed by transfer matrix method. The results show that the number of reflection channels of ISFBG can be tuned for the structure of adjacent sub-gratings with varying index modulations. The reflectivity of main reflection peaks is dependent on the sub-gratings with large index modulations and decreases slightly with the increase of index modulation difference. The secondary reflection peaks are uniformly distributed in the middle of two main reflection peaks and their reflectivity increases with the increment of index modulation difference. Furthermore, the channel spacing of ISFBG can be compressed with channel number increasing by the multiple-phase-shift technique.