A new high resolution reflecting scanning Fourier transform spectrometry produces nonlinear optical path difference (OPD) by rotating two tilted plane mirrors. OPD plays an important role in evaluating the performance and parameters and designing the spectrometry. Based on analyzing the principle of this reflecting scanning Fourier transform spectrometry, we adopt two methods to study OPD. One method is conventional tracing rays method by tracing rays following the Malus law, the other is novel point-image method by using mirror imaging principle and computer simulation to calculate OPD. The general expression of OPD with time change is deduced by the tracing rays method, and factors associated with the change of OPD are pointed out. Factors and their weights on the maximum OPD are also discussed. The calculation results demonstrate that OPD is closely related to the spatial distance between the primary rotating mirror (PRM) and secondary rotating mirror (SRM), the tilted angle between rotating mirrors′ plane and the plane perpendicular to rotational axis, and the incident angle between incident beam split from beam splitter and the rotational axis. Also, it is proved that the tracing rays method and point-image method are both feasible and efficient; and the simulation curves of them agree with each other.