With the merits of simplicity and rapidness, the negative apparent velocity method has been used widely in tunnel geological prediction. The regular method indicates that the intersection of time-distance curves of direct wave and reflected wave is just the position of reflecting interface. However, for inclined interface, the interface position cannot be identified accurately if we only extend time-distance curves. What's more, the more lean the angle, the greater the error. This leads to serious accidents in practice because of inaccurate prediction. Aimed at solving the problem, the authors mainly studied the inclined interface, used the newest coordinates method to get time-distance curve of reflected interface, and then fitted their interpolation, mainly including polynomial fitting curve, unitary three-points isometric interpolation and interpolation by continued fractions. Through a lot of tests and comparisons of the characteristics of all interpolation fittings, the time distance curve interpolation by polynomial fitting can be worked out so as to identify the position of reflected interface accurately. Finally, interface dip (apparent dip) can be calculated using geometric formula. This method facilitates the data processing by means of negative apparent velocity.