Abstract: Broad-band ADCP invented in the early 1990s has improved both distance resolution and speed resolution in contrast to narrow-band ADCP, making it possible to obtain precise measurements of discharge in a shallow river with a low flow speed. This paper focuses on how BBADCP profiles the river by time gating and sampling the received echo from different depths, and how it measures flow speed based on Doppler shift. Using two-dimensional signal vectors, the process of Doppler shift modulation and demodulation is analyzed, and a signal processing method for extracting flow speed information is derived. A BBADCP system model based on base-band simulation and the matched-filter theory is established and the theory is tested with simulation.