In order to solve the problem of depth classification of the underwater target in a very low frequency acoustic field,the active component of cross spectra of particle pressure and horizontal velocity(ACCSPPHV) is adopted to distinguish the surface vessel and the underwater target.According to the effective depth of a Pekeris waveguide,the placing depth forecasting equations of passive vertical double vector hydrophones are proposed.Numerical examples show that when the sum of depths of two hydrophones is the effective depth,the sign distribution of ACCSPPHV has nothing to do with horizontal distance;in addition,the sum of the first critical surface and the second critical surface is equal to the effective depth.By setting the first critical surface less than the difference between the effective water depth and the actual water depth,that is,the second critical surface is greater than the actual depth,the three positive and negative regions of the whole ocean volume are equivalent to two positive and negative regions and therefore the depth classification of the underwater target is obtained.Besides,when the 20 m water depth is taken as the first critical surface in the simulation of underwater targets(40 Hz,50 Hz,and 60 Hz respectively),the effectiveness of the algorithm and the correctness of relevant conclusions are verified,and the analysis of the corresponding forecasting performance is conducted. In order to solve the problem of depth classification of the underwater target in a very low frequency acoustic field, the active component of cross spectra of particle pressure and horizontal velocity (ACCSPPHV) is adopted to distinguish the surface vessel and the underwater target. According to to the effective depth of a Pekeris waveguide, the placing depth forecasting equations of passive vertical double vector hydrophones are proposed. Numerical examples show that when the sum of depths of two hydrophones is the effective depth, the sign distribution of ACCSPPHV has nothing to do with horizontal distance; in addition, the sum of the first critical surface and the second critical surface is equal to the effective depth.By setting the first critical surface less than the difference between the effective water depth and the actual water depth, that is, the second critical surface is greater than the actual depth, the three positive and negative regions of the whole ocean volume are equivalent to t wo positive and negative regions and therefore the depth classification of the underwater target is obtained. When the 20 m water depth is taken as the first critical surface in the simulation of underwater targets (40 Hz, 50 Hz, and 60 Hz respectively) , the effectiveness of the algorithm and the correctness of relevant conclusions are verified, and the analysis of the corresponding forecasting performance is conducted.