Abstract: This article elaborates the moisture measuring principle of capacitor plates which placed in a single plane. The influence of size and space of capacitor plates on the measured depth is compared and researched through theoretical analysis and experiments. The relation between the measurement signal, moisture content and measured depth is obtained with the measurement system. The more accurate measurement of water is obtained by the design, analysis and experiments of hardware and software system. It can predict the location of the coal mine flooding in advance and gain time for evacuation of personnel before flooding accident occurred.