Abstract: The reference sound speed is a key parameter affecting the direction-of-arrival (DOA) estimation of underwater acoustic targets. Due to inherent structural constraints, the horizontal line array (HLA) couples the azimuth and elevation angles into a conical angle, leading to significant azimuth estimation errors when the reference sound speed is not accurately selected. To achieve decoupled, unbiased estimation of azimuth and elevation angles, this work conducts an analysis based on the horizontal plane array (HPA). By employing normal mode theory, the interrelationships among the reference sound speed, azimuth, and elevation angle are derived. The study reveals that azimuth can always be estimated without bias when the reference sound speed is below a certain upper bound. The sine of the elevation angle can be estimated without bias only if the reference sound speed equals the sound speed of seawater at the receiver depth. The simulations and experiments conducted in an underwater waveguide in this work fully validate the above conclusions, thereby providing a theoretical foundation for unbiased DOA estimation of underwater acoustic targets using HPA.