Abstract: Aiming at the topic of source depth discrimination of negative thermocline in shallow water,an approach which is used to extract the waveguide invariant from the interference pattern by warping transform is present.On condition that the receiver is below the thermocline,the waveguide invariant of the surface source is approximate equal to 1 and the waveguide invariant of the underwater source is larger than 1.The interference pattern is warped to obtain its relation items of modes.A time delay pulse which is proportional to the real range is extracted,so the waveguide invariant of the interference pattern can be calculated without knowing the range.This method does not need the nearest passing distance and can be used on a single hydrophone or the LOFAR spectrum which is the output of beam forming by a horizontal array.The formula derivation and simulation results provide that the time delay pulse of the relation items aroused by sources in different depths are proportional to the real range.The waveguide invariant can be extracted and the depth of the source can be identified.At last,the waveguide invariant of a ship is extracted.The experimental results prove its effectiveness.