Adopting synthetic duration curve method and based on the hydrological data of Year 1981-2002, the design water level of part hydrographic stations at the middle and lower reach of the Yangtze River at different guaranteed rates were calculated.

After discussing the time distance relalionship of the reflection wave in the conditions of the two layer level interface and the continuous distribution of the medium speed, the author gives us the equation of the time distance curve of this model.

Corresponding time-distance equations are derived and the time-distancecurves are presented in comparison with the time-distance curves obtained over a horizontallayered earth.

By comparing two calculated time-distance curves based transversely isotropy medium and isotropy medium, respectivly, the validity and accuracy of segmental iteration ray-tracing method are proved and show that it is a effective and practicable ray-tracing method as it is applicable for any .

The least square method is used to build a linear fitting model,and a simple linear fitting is given to calculate v2 through the time-distance curve.

The author analyzed the difference and connection between travel time curves of the refraction wave and reflection wave.