Passive IoT based on enhanced 5G (5G-A) has advantages such as no power restrictions, large connections, wide coverage, and strong computing power, making it suitable for networked monitoring of high value fruit trees and large scale orchards. In this paper, the basic principles and key technologies of 5G-A passive IoT implementation were demonstrated, an orchard monitoring system was designed based on 5G-A passive IoT, and applied it to fruit tree growth status monitoring, growth simulation, and prediction. In terms of system design, using technologies such as 5G-A, data from orchards is collected and transmitted via passive sensor tag arrays. This data is then transmitted through massive machine-type communication (mMTC) network slices. Edge computing nodes are deployed on the 5G-A wireless access network side to perform real-time analysis and computation of the collected data and provide feedback. The corrected WOFOST fruit tree growth model algorithm is then integrated into the fruit tree digital twin system to enable the construction of visualized and dynamic fruit tree growth simulations and predictions. In terms of application, it can collect real-time soil and climate data of fruit tree growth, and count the number of fruit trees and monitor the growth status of each fruit tree; dynamically visualizing the coupling effect between fruit tree growth and environment provides an effective method for precise prediction and scientific planning of its various growth stages. The research results provides theoretical basis and solution basis for the deployment and application of 5G-A passive IoT orchard monitoring system.