This paper discussed the principle of soil moisture monitoring by Frequency Domain Reflectometry (TDR) and Frequency Domain Reflectometry (FDR). Based on the experimental data from automatic monitoring sensors at the Wangchengang Evaporation Experimental Station in Liuan City of Anhui Province, this paper analyzed the precision of automatic monitoring sensors in comparison with the manual sampling data by gravimetric method. A mean of calibrating soil water content calculation equation was further developed using the manual sampling data of gravimetric method. The improved precision of automatic monitoring data was thus obtained.