It is difficult to prepare Fiber Bragg Grating (FBG) with integrated structure of core-and-cladding. This paper first analyzes the structure characteristics of FBG with integrated structure. Based on the optical fiber coupled-mode theory, Rsoft and Matlab are used to simulate the reflection spectra of fiber cores, cladding gratings and integrated gratings respectively. The reflection spectrum characteristics of FBG with integrated structure are analyzed in the simulation with different grating period, grating length and modulation depth. The simulation results show that when the grating period is 541 nm and other parameters are fixed, the central reflection wavelength of the separate fiber core and cladding grating is 1 563.4 and 1 558.0 nm respectively, and the integrated grating can obtain the bimodal reflection spectrum with the same central wavelength. The grating period, grating length and modulation depth of the integrated grating have regular effects on the central wavelength, intensity and half high width of the reflection spectrum. The results can provide reference and theoretical support for grating writing and application.