A method of estimating the source depth of wideband underwater acoustic pulses in deep water is proposed in this paper. The method is based on the characteristic of frequency domain interference oscillation of broadband signals received at a single stationary hydrophone. The oscillation period is related to the Lloyd's Mirror effect (LME) and can be transformed into the information of source depth with the known source range. If the source range is uncertain, one can distinguish the submerged source from the near-surface one according to the sensitivity of estimated source depth versus source range. Experimental results confirm this source depth classification method.