This study aims to construct the estimation method of river network regression coefficient C_s that does not depend on the rainfall-runoff data. Four typical small and medium-sized river basins located in different hydrometeorological divisions are selected in this study. River networks are extracted from the DEM information, simplification schemes for the river network is constructed based on Strahler’s rule, the differential equation for the storage based on the self-similar river network method is improved, and the influence of different river length ratios on the C_s calculation results is analyzed. The results show that: the improved method proposed in this paper can effectively reduce the difference of C_s between the original river network and the simplified scheme to improve the stability; the increase of river network complexity in the basin usually leads to the increase of C_s calculation results; the river length ratio affects the applicability of simplified scheme by influencing the information covered by the river network. For small and medium-sized basins with the 3-level structure, when the length ratio of the level 1 river network is less than 70%, the simplification solution of river network retained to level 2 can be used as an alternative input.