The narrow band filter is used in spatial heterodyne spectrometers to limit the spectral range of the instrument and to avoid spectral overlapping. Due to the change of the filter transmittance with incidence angles, the emergent optical field from the collimating system is non-uniform at different wavelengths. The optical field non-uniformity mechanism is deducted theoretically, proving that modulation of filter causes convolution with a wavelength-related modulation function in restoration spectra and results in variation in full width at half maximum (FWHM) of monochromatic spectrum with wavelength. A simulation is carried out based on the theoretical formulas and the spatial heterodyne spectrometer parameters in laboratory, indicating that there exists influence of filter modulation that appears in concavity at shorter wavelength and convexity at longer wavelength. Monochromatic scanning experiments are performed in laboratory, and the measurement results are consistent with those of theoretical analysis and simulation. An enveloping line fitting correction method is proposed. The variation in FWHM of the instrument line function decreases from 16.7% to 0.2% after correction, suggesting that the influence of modulation function of filter on restored spectra is eliminated effectively.