Abstract: Based on the coupled-mode theory, the influences on coupled precision in characteristic calculation for different modes in long period fiber grating are analyzed. Calculated results show that for the low order cladding mode, the longitudinal coupling constants between the odd order cladding mode and the first core mode are smaller than that between even order cladding mode and the first core mode. So we can ignore the influence of odd order cladding mode in calculation without reducing the calculation precision. At the same time, we can select the suitable orders of sinusoidal index modulation to reduce the work of calculation according to the arrangement of wavelength and calculation precision.