A ultra long period fiber grating (ULPFG) is a novel band-rejection fiber device with periods up to several millimeters. Some related devices have been used in measurement of multi-parameter, but it′s reported very few on how to simulate its transmission spectrum properties. On the base of coupled-mode theory and simplified three-layered model, a numerical method is proposed to simulate the transmission of the ULPFG written in single mode fiber and the linear interpolation is used to simplify the calculation. By using this algorithm, the calculation resonant wavelengths generally agree with the experimental results previously reported, which verifying the validity and reasonability of the algorithm and theory basis and effective analysis method for the witting providing ULPFG.