Abstract: This paper analyses and summarizes the previous methods determining the source/sink functions or bed-boundary conditions of transport equation of suspended sediment.There are some limitations in these methods due to introducing the concept of equilibrium transport and ignoring the effects of bed-load on suspendedload transport when considering the mass exchanges near the bed. The theory about mass exchange rates of sediment near the bed previously developed by writers is adopted here to make an inquiry into appropriate boundary conditions on the bed, so that the general equation of suspended sediment transport can be solved. The processes of the adjustment of sand concentrations in one-dimensional or vertical two-dimensional flows have been computed and compared respectively with the measured data, showing reasonably good agreements.