Fourier transform infrared (FTIR) spectroscopy Si―OH group is widely used in Si―OH content meas-urement of fused silica optics. However, the measurement accuracy is influenced by water molecular absorption bands in low Si―OH content samples and absorption saturation in high Si―OH content samples, leading to lim-ited measurement range. FTIR spectroscopy is employed to measure 2500~5000 cm^-1transmittance spectra of fused silica samples with different OH contents and thicknesses. The interference of water molecular absorption band to 3673 cm^-1is eliminated. Then Si―OH contents, corresponding measurement errors and limits of detec-tion at 3673 cm^-1and 4522 cm^-1bands are calculated. Based on the experimental results and Beer’s law, a model to correlate Si―OH content, sample thickness, measurement error of transmittance, and measurement error of Si―OH content is established. From this model, by using 3673 cm^-1band for fused silica samples with Si―OH content less than 8.17×10^-4and 4522 cm^-1band for samples with Si―OH content more than 8.17×10^-4to meas-ure the Si―OH content, a dynamic range from 0.4×10^-6to 10^-2Si―OH can be achieved with optimized accuracy for fused silica samples with 2 mm thickness.