Based on the concept of effective refractive index and the power density distribution of cladding mode, this paper re-survey the phenomenon of "mode conversion" of long-period fiber grating (LPFG), and gives new physical image for "mode conversion". By modifying the range of the effective refractive index of cladding mode of LPFG, it is pointed out that the step-like growth of the effective refractive index is the self-property of each cladding mode and there is no replacement between high and low order modes, and we rename the "mode conversion region" to "mode barrier region". It is pointed out that the incorrectness of the argument that the lower order cladding modes will guided in the overlay in the "mode transition" region by analyzing the power density distribution of cladding mode. The research in this paper shows that when the thickness of overlay reaches a certain value, the power distribution of the cladding will be more concentrated in the overlay, but the cladding mode will not be guided in overlay. This paper discusses the optimization of overlay parameters of long-period fiber grating sensors, and compares the optimization results before and after the modification of effective refractive index of cladding mode, the result shows that there is a large deviation between the optimization results of these two cases. The correctness of the optimization results after modifying the effective refractive index range is explained mechanistically, which provides the new theoretical guidance for designing high-sensitivity long-period fiber grating sensors.