Abstract: No lithostratigraphic unit has been set up for the Silurian-Devonian strata in the vicinity of the Jinsha River,western Sichuan. Through sorting stratigraphic units,the author found that this sequence of strata has clear contact relationships with its overlying strata,shows distinct lithologic features and contains certain fossils,so it meets the conditions for establishing a lithostrtqtigraphic unit.So with the Ranxi section of the Baiyushan Rock Formation measured by predecessors as a holostratotype section,a normal formation──the Ranxi Formation──was estab-lished for the above-metioned sequence of strata. This paper describes the stratotype section of the Ranxi Forma-tion.