Water plays an important role in various physiological activities of plants. Measuring plant stem water content in real time without damage has important guiding significance for studying the life activities of plants. A kind of stem water content of realtime and nondamage online detection method based on standing wave ratio was introduced in detail. BD-IV stem water content sensor was designed based on the method. With organic solvents and the beakers as the simulation environment, pine as the experimental materials of the section of tree, and peach as experimental materials of standing tree, the detection performance of BD-IV stem water content sensor was verified. BD-IV stem water content sensor can accurately detect volume water content under the conditions of plant stem diameter between 5.0cm and 10.5cm, volume water content between 1.17% and 59.59%. Through the longterm monitoring of daily and monthly changing rule of stem water content of peach, the result measured by BD-IV stem water content sensor was consistent with plant physiological characteristics. Through the longterm monitoring of stem water content and stem sap flow of willow by using BD-IV stem water content sensor and SF-L stem flow gauge made in Germany, the relationship between stem water content and stem sap flow was summarized. It provides a localization testing instruments with high performance to price ratio for the research on plant physiological characteristics.