The stem moisture status of living tree is an effective manifestation of plant life state. Stem water content (StWC) and stem sap flux density (SFD) are important parameters to study the variation of water in plants. Stem water content is a fundamental parameter to correctly detect the thermal equilibrium point or zero-flux conditions and measure the sap flux density. The water content at different heights and the sap flux density in different orientations of the stems of the living tree may differ significantly. The plant growth status can be evaluated comprehensively and the relationship between the water content and sap flux density can be analyzed effectively with accurate detection of the two parameters at the same spatial position of living tree stem. The stem water content detection method based on standing wave ratio (SWR) principle and the stem sap flow detection method based on heat ratio method (HRM) principle were combined to design a composite detection system for stem water content and sap flux density of living trees. The water content detection unit and the sap flow detection unit of the composite detection system reused one set of three-needle probes, which could accurately detect water content and sap flow in the same spatial position of the living tree stems in real time. The output voltage of the water content detection unit had a good linear relationship (R2=0.9701) with the dielectric constant (in the range of 6～53.3, corresponding to the stem water content range of 0～85%), and the static stability was good (with maximum fluctuation of 0.6% of the full scale for a long time test). The measuring results of the water content detection unit and BD-IV plant stem moisture sensor were consistent（R2=0.9800）in a comparative test taking poplar as the research object. The comparative test between the sap flow detection unit and the ST1221 thermal dissipation plant sap flow meter showed a highly significant linear relationship between the value of sap flux density detected by both (R2=0.8991), and the mean value of sap flux density detected by the ST1221 sap flow meter was 1.1cm/h lower than that of the sap flow detection unit, mainly because the thermal dissipation sap flow meter could not accurately determine the zero flow conditions leading to its underestimation of sap flux density, while the heat ratio method used by the sap flow detection unit can accurately detect low-speed sap flow. The long-term monitoring results of poplar stem water content and sap flow by the composite detection system were consistent with previous studies and in line with plant physiological laws. There was a significant negative correlation between stem water content and sap flux density (Pearson correlation coefficient was -0.7951). A high-performance and low-cost device for plant life state monitoring was provided.