Field Of View (FOV) is an important contributor for instrumental line shape (ILS) function of high-resolution Fourier transform infrared spectrometer (FTS) only next to maximum optical difference. For the reason of optical design and layout, commonly, the measured FOV located in detector is not regularly rounded as original one. There exits more or less difference between the measured vertical FOV and measured horizontal one. In view of this case, the present paper replaces the generally circular area light source with an elliptical one, which is probably more suitable for actual FTS. At the same time, the factor maximum optical difference was considered. After these, the mathematic and graphic description about ILS function with unequal field of view （UFOV） was given. Finally, comparison between measured spectrum with standard monoxide gas and calculated one with equal field of view (EFOV) and UFOV respectively was taken. Experimental results show that the spectral residual from UFOV is less fluctuant than those from EFOV and its root mean square value is the smallest. All these indicate that the ILS function from UFOV is more accurate and suited to reflecting the response of high-resolution FTS than the one from EFOV.