Abstract: A novel approximate formula of time-domain reflection coefficient is put forward.The form is simple and its precision is not affected by the truncation errors.In addition, the form has the self-contained analytic formula and exact result.The average power delay distribution is predicted by this method for indoor ultra-wideband(UWB) signal. Good agreement is achieved between the simulated results and the published results. It is proved that the novel method of time-domain ray tracing is effective to predict the propagation characteristics of UWB signal.