The cross-spectrum characteristics of sound pressure between two hydrophones deployed vertically are analyzed in low frequency acoustic field of shallow water, more attentions are paid to the cross-spectrum active component of sound pressure when only two normal modes are trapped in the waveguide. The numerical results show that （1） the sign of the sound pressure cross-spectrum active component is independent of range when receiving transducers placed in appropriate depths, so it can be used to tell whether the sound source is near the surface or underwater by selecting a pair of appropriate deploying depth of transducers; （2） acoustic velocity profiles have little effects on the application of this method, the seabed attenuation will limit the detection range, but the algorithm still has a good robustness in the valid detection range. Approximate theoretic analysis of cross-spectrum active component characteristics is proposed in the paper combined with the concept of effective depth, and the prediction formula of appropriate deploying depth of transducers is presented. The theoretic analysis indicates that the sum of the bi-hydrophone depths should be equal to the waveguide effective depth and the sign distribution of the sound pressure cross-spectrum active component is divided into three level regions in an ideal pressure-release waveguide. The theoretic analysis provides deeper understanding of the low-frequency sound field characteristics in shallow water and also guidance to the application of the algorithm.