In the infrared spectroscopic transmittance testing system based on the retro-reflective method, the position of the components such as aperture will affect the measurement of spectral transmittance of the infrared lens during measurement. To address this issue, a vignetting model was established to analyze the vignetting of the measurement beam in the testing system, and the vignetting formula generated by aperture and other components placing at different positions was derived. Experimental results show that maintaining the consistency in relative vignetting between the aerial and actual measurements can effectively reduce the effect of vignetting variations on the measurement of spectral transmittance of the infrared lens.