Si―OH group is the most common impurity in fused silica, which has a significant impact on the optical perfor-mance of fused silica optics from infrared to ultraviolet spectral regions. The Si―OH content can be directly meas-ured from Si―OH stretching band at 3673 cm^-1with molar absorptivity of (77.5±1.5) L/(mol·cm). However, meas-urement range is limited by interference of absorption bands from water molecule in low Si―OH content samples and absorption saturation in high Si―OH content samples. Fourier transform infrared (FTIR) spectroscopy is em-ployed to measure 2500 cm^-1~5000 cm^-1transmittance spectra of fused silica samples with different Si―OH con-tents ranging from 0.0 to 10^-3and thicknesses ranging from 2.0 mm to 8.0 mm. Since 3000 cm^-1~3100 cm^-1absorp-tion bands from H²O molecule is not covered by other infrared (IR) absorption bands, absorption at 3673 cm^-1from H²O molecule can be eliminated using transmittance from 3000 cm^-1~3100 cm^-1region and line shape of H²O band obtained by Gaussian fit from low Si―OH content samples. Compared with 4522 cm^-1absorption peak which is not interfered by any absorption bands, the Si―OH content calculated from 3673 cm-1 band without influence of water absorption band has a relative error about 2.5% lower than that directly calculated by 3673 cm-1 band in Suprasil 501 sample with about 5.0×10^-5Si―OH content and 2.0 mm thickness. After eliminating the influence of water ab-sorption band, corresponding measurement errors and limit of detection for Si―OH content at 3673 cm^-1and 4522 cm^-1bands are calculated. Based on experimental data 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, Si―OH content measurement error is mainly influenced by logarithm of transmittance and influence of water absorption band in low Si―OH content samples, and absorption saturation in high Si―OH content samples. Since the molar absorptivity at 4522 cm^-1is nearly 50 times weaker than that at 3673 cm^-1, the corresponding transmit-tance at 4522 cm^-1is much lower than that at 3673 cm^-1, leading to a lower relative root mean square error for Si―OH content measurement at 4522 cm^-1than that at 3673 cm^-1in high OH content samples. With 2.0 mm thick-ness, a relative error less than 0.13% can be achieved in sample with 10^-2Si―OH content at 4522 cm^-1absorption band. As a result, by eliminating water absorption band at 3673 cm^-1for low Si―OH content samples and employing 4522 cm^-1band for high Si―OH content samples to measure the Si―OH content, the measurement range is increased from (6.0~1810.0)×10^-6to (0.4~10000.0)×10^-6at 2.0 mm thickness with improved measurement accuracy.