The spatial and energy resources available on small underwater platforms are limited, imposing strict restrictions on the structure, weight, and channel count of the sonar array they carry. To enhance the direction of arrival (DOA) estimation capabilities of small platform sonars under these constraints, this paper examines a combined vertical array (CVA), composed of a single vector hydrophone and pressure hydrophones. The vertical line array is insensitive to azimuthal variations, but the CVA is capable of horizontal direction estimation with better gain compared to a single vector hydrophone. A novel DOA estimation algorithm, based on the joint processing of sound pressure and particle velocity signals from a CVA, has been proposed: By jointly processing the sound pressure signals from the vertical array and the particle velocity signals from a single vector hydrophone, the method achieves gain transformation of the CVA in three-dimensional space, effectively enhancing the horizontal bearing estimation capability. Expressions for the horizontal gain of the CVA were derived, and the impact of array and signal parameters on the horizontal direction estimation accuracy of the CVA was simulated using the Monte Carlo method. Numerical simulations and experimental results indicate that the horizontal direction estimation accuracy of the CVA surpasses that of traditional vector hydrophones, with high-frequency gain stability unaffected by grating lobes in the elevation direction. This allows for high-precision, unambiguous estimation of target directions.