In this article, an attempt is made to apply the catastrophe theory to soil erosion study. Xin Gou county is chosen as an example. There soil erosion evolution is depicted and its characteristics are explained with the theory of catastrophe. For quantitative study, a cusp catastrophe model is used to simulate the evolution process of soil erosion.On the basis of the model, some characteristics of soil erosion can be pointed out. They are catastrophe, binodality, hysteresis and divergence. A classification is made according to differnt erosion states, namely unstable erosion state, stable erosion state and the state with potential catastrophic danger. From this point of view of catastrophe theory, under the conditions of limeted labour and funds, the soil conservation measures should be focused in the areas where the soil erosion degree belongs to the light and medium categories.