This paper introduces a method for measuring water content of crude oil based on the principle that the dielectric constant changes of oil-water mixture can impact the antenna radiation characteristic. By the empirical formula of the mixture dielectric constant and the antenna radiation characteristic, this paper deduces the relation between antenna equivalent input impedance and dielectric constant in open circuit, finally obtains the relation between the input impedance and water content. The relation between the output voltage and water content will be found by measuring the voltage at the equivalent input impedance, while realizing the water content measurement. Using the measurement method of stritifield scanning to the man made sample then calculating average to improve measurement accuracy. Compared with the existing methods, this new method, which takes on certain significance for the engineering practice, has the characteristics of simple operation, short time period, overcoming the impact of electronic conductivity of crude oil and so on.