Abstract: The oxygen content in the soil of tree root zones is a crucial environmental factor influencing the growth of trees. Due to the complex spatial distribution of oxygen, direct measurement has significant limitations. This study developed a composite parameter detection sensor for oxygen content in tree trunk xylem and sap flow rate based on the principles of fluorescence quenching and thermal diffusion sap flow measurement. The sensor was calibrated, temperature - corrected, and its performance was tested. The aim was to explore the relationship between the oxygen content in the water transfer path and the oxygen content in the root - zone soil, with water transfer as the key breakthrough clue. The calibration tests showed that when the oxygen content was lower than 21%, the relative error of oxygen content detection was less than 1.34%, and the relative error of sap flow rate was less than 5%. Outdoor control tests indicated that the correlation coefficients between the detection results of the composite sensor and commercial fiber - optic oxygen sensors and sap flow rate sensors were 0.947 and 0.958, respectively. By combining the monitoring data of the composite sensor and micrometeorological parameters, and conducting in - depth analysis using the tree macroscopic water - uptake model and the root - soil oxygen diffusion model, the distribution characteristics of the oxygen environment content at different root depths were derived, enabling effective monitoring of the oxygen environment content in tree roots. This research can provide technical support for the dynamic monitoring of the oxygen environment in tree roots, and also offer a scientific basis for optimizing the tree growth environment and enhancing the water - oxygen management of trees.