Abstract: For the variable-gap Fabry-Perot (F-P) interferometric imaging spectrometer, whether the spatial interference information is sampled at equal intervals of optical path difference (OPD), that is, whether the relationship between optical path difference and detector pixel position is linear, directly determines the difficulty of spectral data processing . After analysis, the OPD of the variable-gap F-P interferometric imaging system was the function of the centre distance of two Airy disks produced by the beam split of the interferometer. However, the centre distance of two Airy disks did not always maintain a linear relationship with the detector pixel position, which was related to the value of the wedge angle of the interferometer. The relationship between optical path difference and detector pixel position under different wedge angle conditions was studied. The results show that there is a good linear relationship between optical path difference and detector array pixel position at any wedge angle, and the position contact ratio of characteristic peak measured by long wave infrared spectral imaging system with variable-gap F-P interference and spectral radiometer with high precision reaches to 100%.