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Water is directly or indirectly involved in the activities of plant physiology. One role of stems is water storage, the other is water transmission channel. Detection of plant stem water by non-destructive online method becomes one of the hot spots in the study of plant water physiological activity.A real-time online non-destructive detection system designed by LabVIEW was proposed based on ultrasonic echoes, which was composed by ultrasonic probe,，ultrasonic RF transmitter receiver, data acquisition instrument and portable computer. The simulation model was built by the system with various ultrasonic velocities of different organic solutions to represent the water content of plant stems. The correlation between the changes of water content and that of ultrasonic velocity was analyzed on a set of poplar cutting samples in the process of water absorption of those samples. The average determination coefficient reached about 0.98. For 24h detection on stem of a living Michelia alba, the highest ultrasonic velocity appeared at 14:00, and the lowest appeared at 22:00. The results showed the variations of ultrasonic velocity accorded with plant physiological characteristic. Meanwhile, the variations of velocities in radial and axial directions and in different kinds of plant stem were detected by the system, which provided a detection instrument with structural differences for the research on plant stem water physiological characteristics.