Abstract: The Acoustic Doppler Current Profiler (ADCP) is an important instrument for hydrological measurements and is widely used in oceanography, hydrology, and environmental science. In practical applications, deviations arising from manufacturing processes and installation can introduce errors into measurement results. This paper addresses ADCP beam calibration and proposes a multi-angle, still-water, straight-line navigation calibration method based on the Global Navigation Satellite System (GNSS). Systematic bias is eliminated by computing the difference between forward and reverse track velocities, and the projection equation is constructed using multi-angle observations. The least squares method is employed to estimate the beam geometric deviation angles and the correction weight matrix. This calibration approach reduces the error impact caused by manufacturing imperfections and installation misalignments, lowering the ADCP’s tracking error from 6.28% to 1.3‰.