Abstract: This study focused on the characteristics and mechanism of ultrasonic acoustic emissions (UAEs) from the cavitation in xylem sap. A bubble motion equation based on the RayleighPlesset equation was established and solved. The UAEs from xylem were detected by constructing an ultrasonic detection system and then the equilibrium radius was calculated. By solving the equation with a fourorder RungeKutta method, we found that the radii of the bubbles were greater or less than the equilibrium radius when disturbed by other changes, such as changes of xylem sap pressure; the bubbles would get back to the stable equilibrium by the way of convergent oscillation, which was the source of UAEs. UAEs from slices of Platycladus orientalis, Acer truncatum and Robinia pseudoacacia when losing water were detected and analyzed. The results showed that the frequency and ringing counts of the signals detected had no significant correlation with conduits or tracheids. About 80% of the signals detected were similar to that calculated. The tiny air bubbles in conduits or tracheids emited UAEs when they were expanded and contracted by the way of convergent oscillation.