By measuring the AC impedance spectra of mortar specimen through a wide range of frequency, linear relationship between the capacitance in parallel in the equivalent circuit at high frequency range and the water content of the mortar specimens can be established. This relationship enables us to know the water content of the specimens. As the water content decreases, the semicircle of the complex plane at the middle and low frequency range will split. It is the result of the change of the capillary structure of the mortar specimen.