Based on the strict four-layer model, the modes reorganization of the coated long period fiber gratings (LPFG) appearing in the increase of film thickness is studied in details. The result shows that the periodic reorganization of coated LPFG occurs when the film thickness increases, and in each reorganization, a pair of adjacent cladding modes go into the film layer, guided by the thin film layer. But the cladding modes do not enter the thin layer synchronically. Every time HE modes go into the thin layer first, and pass some degree of thickness, and then it′s time for EH modes to enter the thin layer. In the process of reorganization, alternating current (AC) coupling coefficient varies greatly with the change of layer thickness. When HE modes go into the thin layer, AC coupling coefficient declines sharply; while EH modes go into the thin layer, AC coupling coefficient rises rapidly. Corresponding with the two adjacent modes in turn into the film layer, two jumps of the resonant wavelength of the coated LPFG occur in the process of reorganization. The research also indicates that, in reorganization, the refractive index sensitivity of the coated LPFG increases obviously, compared with the time before it is reorganized.