The choice of a quantitative conversion model is a key to the ¹³⁷Cs measurement to estimate soil erosion rate.Two main kinds of models including preliminary empirical equations and recent theoretical models have been established. The ranges and limitations of the common models are discussed respectively to the mass-balance model.Great attention is paid for its precision and basis of physical-chemical processes.At the same time,there are also several limitations with them.Firstly,for the convenience of computation,the processes are simplified.Secondly,in order to improve the processes related to the ¹³⁷Cs precipitation and erosion,many parameters which can not be measured accurately are incorporated.The profile-distribution model and the diffusion and migration model have the same situations.The results from the same reduction of ¹³⁷Cs may differ remarkably between different models for their different theories and expressions.So,improving the existing models and developing more reasonable models are still one of the main tasks for the ¹³⁷Cs technology.