The measurement of water vapor in gas is very important for many industrial production and scientific research fields. With the more stringent test requirements of gas moisture content, the analysis and measurement methods are also required to be more sensitive. At present, the instrument methods for measuring nmol / mol trace water include tunable diode laser absorption spectroscopy, optical cavity ring down spectroscopy and atmospheric pressure ion mass spectrometry. In addition, the measurement techniques of sensors reaching this trace level include quartz crystal microbalance method, cold mirror dew point method, resistance capacitance method and electrolytic method. These approaches are reviewed. The advantages and disadvantages of these methods as well as the future development trends are discussed. It is pointed out that the measurement accuracy of trace moisture content can be improved to a certain extent by overcoming the interference factors in the measurement process. It is beneficial to promote the development of accurate measurement technology of trace water content.