Erosion loss is one of the most important problems in the ecological system. For the heavily silt loaded river, it is necessary to allocate a certain amount of ecological water demand for the maintenance of the ecological system balance. Taking the heavily-silt loaded river as the research object and based on the sediment transport kinetic theory, the river sediment discharge and sand carrying capacity were analyzed in this paper. Therefore, the calculation method for the critical river flow rate in the no scouring and no sanding status was proposed in this paper. This method is exactly the calculation method for the lowest ecological water demand to prevent the river sediment accumulation. Using an actual example, the application of the proposed calculation method was carried out and illustrated. It is noteworthy that this study will be beneficial for water resources proper allocation and ecological balance maintenance.