For an ion beam sputtering(IBS) system， the depositon rate is very stable and the reproducibility is excellent，but at the beginning of a deposition the rate may still change. In order to study the change rule， mid-infrared reflectors are prepared by using Nb2O5 and Al2O3 as high and low refractive index materials. Fourier-transform infrared spectroscopy is used to measure the reflection spectra， and the film’s secetion topography is detected by a scanning electron microscope(SEM). The results show that the thickness decreses as the layer number increases， indicating the decreasing deposition rate with time. By spectra inversion， an exponential changing model is established. After optimization， the fitting accuracy between theoretical curve and measured data is greatly improved. This study can be used to correct film thickness in a time-rate monitoring system.