The quality and performance of mineral oil used in the industrial production is closely related to oil trace moisture content. Excess moisture contamination would lead to Oil degradation and aggravating the corrosion and wear of the equipment. So moisture content is one of the routine testing of industrial oil. Accurate online monitoring of moisture content in oil is particularly important whether before using or in using. Analog the definition of water activity in food, This paper deduced the definition of water activity in oil according to related principles of equilibrium thermodynamics. This paper established a link between water activity and relative humidity and convert the measurement of water activity to the relative humidity measurement. This paper expounded the science and rationality of using water activity to describe the moisture content in oil by quoting the concept of saturation margin. And proposed a trace moisture measurement in oil base on water activity. The result shows that this measurement method has good has good precision and can describe the changes of moisture content in oil effectively.