The sediment in the landslide-dammed/check dam reservoirs on the Loess Plateau of China is the direct result of the erosion and retransfer of loess and is characterized by high deposition rate and lots of deposition couplets, thus it is an ideal object to trace erosion flux and understand the processes of the Earth’s Critical Zone. How to exactly identify and plot out the couplet is the basis for using the sediment sequence to recover soil erosion and eco-environment change within a small catchment. In this study, Jingbian (paleo) and Heshui landslide-dammed reservoirs in the north of Loess Plateau were choose as the study sites based on field investigation and documents records. Meanwhile, the physical, geochemical and biological characters of the representative couplets were analyzed in order to comprehensively understand the feature of the couplet and further put forwards the effective geochemical identification index. The results showed that the lower coarse-grained layer in the couplet was composed of coarse silts and fine sands, which was characterized by low water content and high luminance, enriched with quartz and zircon, along with high Si and Zr. On the other hand, the upper fine-grained one was characterized by high contents of clays and fine silts, high water content, low luminance and lenticular bedding, correlating well with high contents of calcite, muscovite, illite, chlorite and kaolinite, along with high Al, K, Fe, Rb, Ca, and Sr. Pollen, organic carbon and nitrogen, phospholipid fatty acids, microorganism also concentrated in this layer. Then, Rb/Zr ratios by XRF core scanning are recommended as the ideal geochemical index to identify deposition couplets within the sediment sequence. These results provide the scientific foundation on deeply realizing the processes of erosion and weathering, hydrology and eco-environment change in the Earth’s Critical Zone on the Loess Plateau.