Abstract: TDR was used to determine dielectric constant of THF aqueous solution with different THF/H₂O mole ratios before and after THF hydrate formation. As a result, the empirical formula was built to measure the water content of the system of "THF aqueous solution + THF hydrate". Then, the formula was applied to calculation of the content of THF hydrate produced by THF aqueous solution and to establishment on correlation between the content of hydrate and dielectric constant. In addition, the dielectric constant decreased during hydrate formation in THF aqueous solution.