Abstract: A theoretical analysis of designing an acoustic back-scatter system for observations of suspended sediments has been made through a sonar equation derived in this paper. Some formulae and data for reference in determining several main technical parameters of the system, such as optimum frequency, sound transmission power, aperture of transducer etc., are provided. And some key problems on the acoustic observation of suspended sediments, such as data processing of a time-varying stochastic process, compensation of sound propagation loss in complicated conditions and in-situ calibration of sediment concentration, are discussed in detail. The results of the analysis could be helpful to the development and application of a specific acoustic back-scatter instrument (e.g. the acoustic sediment concentration profiler, the acoustic Doppler current profiler etc.).