Abstract: Ultrasonic guided wave can reflect the material properties of the long bone. The S transform with high-resolution features was proposed to extract the feature velocity in ultrasonic guided wave, and then to reflect the mechanical properties of long bone. Firstly, S transform was performed on the long bone guided wave. Then the maximum method was used to determine the propagation time corresponding to maximum energy extreme point in time-frequency domain. According to the propagation distance, the feature velocities of different modes in ultrasonic guided wave were calculated. The simulation results show that the feature velocities are linearly correlated to bone Poisson's ratio, and is insensitive to the thickness of cortical bones. Therefore, it can be used to evaluate the bone Poisson's ratio. In addition, the method does not rely on the prior knowledge of the guided wave so that it has a certain practical value.