Abstract: Under uniaxial multi-level loading condition， the longitudinal wave velocities in different directions from south Fujian diorite were measured.Firstly， based on the experiment results， the changes of wave velocities with stress were analyzed. And then according to the hypothesis of the linear relationship between wave velocity and stress in a very small range of stress， a wave velocity-stress model was built.In the end， using the multi-parameter piecewise fitting， undetermined coefficients in the model were determined. The results showed that the variation of wave velocity paralleled to the loading direction is characterized by an initial instant and nonlinear rise below a critical stress， followed by a more gradual linear increase above this critical stress， and the decrease is not obvious before the failure. Whereas， the variation of wave velocity perpendicular to the loading direction is characterized by a slightly rise below a critical stress， followed by an accelerated decrease above this critical stress， and the decrease is very obvious before the failure. The variation of wave velocities in different directions can be accurately fitted by the wave velocity-stress model， which confirms the applicability of the model.

Key words: uniaxial multi-level loading, wave velocity, rock failure, crack evolution, wave velocity-stress model