Cs is an important radionuclide in the environment. It is produced by nuclear test in the atmosphere and nuclear accidents, then existed and accumulated. 137 Cs are used in many fields of environmental geochemistry. The behavior of 137 Cs at high content has researched much more. But pay little attention to the geochemistry behavior of 137 Cs at environmental background level because of the difficulties of research methods and experiment design. For the extensive usage of 137 Cs at several thousand becquerels as a tracer in the soil erosion and lake and reservoir sediment research, these use need detailed understanding of the environmental behavior of 137 Cs. The results showed that: (1) the source and fallout of 137 Cs with time in soil body. (2) the adsorption and fixation in soil composition. (3) input and output of 137 Cs in soil. In the end, the theory of tracing soil erosion and technical path of quantifying erosion using 137 Cs are discussed, the advantages of 137 Cs as a tracer compared to the other radionuclide are concluded.