The soil moisture is the most direct and important index to reflect the drought, soil water is the direct source of theplant water, deciding the plant growth condition, so the measurement of soil water has important practical significance. In recentyears, along with the drought relief developing, soil moisture monitoring has been paid attention on, and automatic instruments forsoil moisture monitoring have been gradually used. Because of later creation of the soil moisture automatic monitoring in water re -sources departments, the people have not enough understanding of the working principle. Based on introduction to soil moisturemonitoring technology, this paper described in detail the working principle of the Frequency Domain Reflectometry (FDR) and TimeDomain Reflectometry (TDR), and the relevant mathematical formulas. The advantages and disadvantages of the monitoring technology were reviewed, and some suggestions were made for the future