Abstract: Water vapor content is one of the important parameter of a Sin-glet Oxygen Generator （SOG） in researching of ChemicaI Oxygen-Iodine Laser（COIL）.The water vapor will not only quench the O₂（¹Δ）,but also it will seriously quench the exited I^* in COIL cavity. Evidently, the water vapor content at the exit of SOG must be measured and controlled. A spectrometric method can successfully meas-ure the water vapor content in SOG by using the special function of OMA-3（optical multichannel analyzer） to finish scanning and accumulating for spectra in differ-ent measuring points at the same time. The measuring precision is about ±10%.