[Objective] As a key parameter in the simulation study of ecosystem water cycle process, the rapid dynamic simulation of the Leaf Area Index (LAI) can solve the limitation that the coupled soil water-vapor-heat-air model STEMMUS (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil) can only use fixed or measured as an input parameter. [Methods] In this paper, the "plant leaf area development sub-module" in EPIC model was coupled with the STEMMUS model, and the model was calibrated and validated using the measured fruit tree transpiration, soil moisture and soil temperature data under the apple growth conditions in the mountain apple Experimental demonstration base of Zizhou County in 2019 and 2020, so as to evaluate the applicability of the coupled model on the Loess Plateau. [Results] The results showed that by optimizing the plant growth parameters, the coupled leaf area development sub-module of the STEMMUS model significantly improved the simulation accuracy of the transpiration and water consumption process of apple trees, with the normalized root mean square error (NRMSE) of the calibration and validation years increasing from 40.2% and 61.9% in the original model to 30.0% and 33.2% in the coupled model, and the mean absolute error (MAE) increasing from 0.52 mm d-1 and 0.64 mm d-1 to 0.42 mm d-1 and 0.38 mm d-1. Meanwhile, the coupled model can better simulate the soil hydrothermal dynamic processes in apple orchards, and the NRMSE of simulated soil water content and soil temperature at the calibrated period and validation period ranged from 1.4% to 32.9% and 2.9 to 9.5%, and the MAE ranged from 0.13 to 4.26 cm3 cm-3 and 0.34 to 1.49°C, respectively. [Conclusion] The high agreement between the simulated and measured values indicates that the coupled model can accurately describe the dynamic growth process of fruit tree leaf area and the ecohydrological process of orchard in the Loess Plateau, and this study can provide technical support for the research of the ecohydrological process of orchard in Loess area.