The linearity of optical path difference (OPD) in image plane interference imaging spectrometer (IPIIS) is directly related to the rebuilt accuracy of the spectrum. The generation principle of OPD is studied. By analyzing the OPD of positive defocus and negative defocus of one single image plane separately, the expression of OPD in both single-side defocus and dual-side defocus working mode of IPIIS is given. The simulation result of OPD and its nonlinear residual in the two aforementioned working modes when Nyquist constraint is satisfied for different wave-lengthes is also given, which shows that both the OPD and its nonlinear residual increase with the angular value between two image planes. The OPD on both sides of the zero-order fringe is dissymmetried in single-side defocus working mode and the negative defocus has small residual. However, there is symmetric OPD in dual-side defocus working mode and has more residual than that before. The experimental results show that the OPD of negative defocus in single-side defocus mode, when the IPIIS works in relatively short wave-length (such as ultraviolet), can be regarded as linear distribution in practical application.