To investigate the feasibility and quantify performance metrics of AlGaN/GaN high-electron-mobility transistor (HEMT) heterodyne detectors in the domain of terahertz direction of arrival (DOA) estimation, this study benefits from the high bandwidth and resolution characteristics of terahertz waves, as well as the process consistency and scalability of AlGaN/GaN high-electron-mobility transistor terahertz heterodyne mixers. A 243 GHz linear array vector detection system based on these mixers was designed and constructed, characterizing its various performance attributes.This paper establishes a 243 GHz linear array vector detection system based on an AlGaN/GaN HEMT mixer linear array (Fig.1(a)). The transmitted terahertz waves to be measured are collimated by an off-axis parabolic mirror (OAP). The receiver uses a high-speed multi-channel ADC array to collect signals from the mixer array, which have been amplified by an intermediate frequency signal amplifier, for processing in the higher-level computer. The processing flow (Fig.2(b)) includes amplitude and phase resolution, calibration, and beamforming.Testing demonstrates that the system's phase resolution stability is superior to 0.6°, with this error primarily stemming from the phase noise of the microwave source. Future use of a microwave source with lower phase noise and increasing the frequency of the system's intermediate frequency signal for heterodyne detection could further reduce this error. The system exhibits a phase distribution detection relative error of less than 3.6%, attributed to minor discrepancies introduced during the patch assembly of each channel's mixer. Employing more precise assembly techniques in the future could decrease this error. The system's error in detecting terahertz waves arriving from ±11° of the normal direction is less than 0.25°, and the normalized level of the first sidelobe is around -3 dB, due to variations in the response consistency of each channel's mixer. Enhancing the consistency of mixers across channels or increasing the number of elements on the linear array could further reduce sidelobe levels and improve the accuracy of direction of arrival detection. The system's field of view reaches 22°, a result of the current 3 mm spacing between linear array elements leading to insufficient spatial sampling rates for terahertz waves and a limited field of view constrained by the 3 mm superhemispherical silicon lens used to couple the measured terahertz waves. Future efforts will focus on reducing the spacing between linear array elements and adopting a more optimal coupling method for the measured terahertz waves to enhance the overall field of view of the system.This paper establishes a 243 GHz terahertz linear array vector detection system characterized by high phase resolution stability and low error in the direction of arrival detection. By comparing test results with simulation outcomes, it was found that the system's phase distribution detection error is less than 3.6%, the error in detecting terahertz wave direction of arrival within ±11° of the normal direction is less than 0.25°, and the field of view reaches 22°. These results validate the feasibility of applying AlGaN/GaN HEMT heterodyne detector linear array components in the field of terahertz DOA estimation. This lays the groundwork for the subsequent development of terahertz phased array radars and directional communication systems with larger fields of view and smaller system sizes.