This paper deals with the soil structure degradation characteristics and its possible mechanism in Udic ferralisols. Results showed that compared with the soil under zonal natural vegetation it suffered more structure degradation under secondary or man-made vegetation. It was found that the structure degradation in Udic ferralisols was characterized by sandification of surface soil, lowering of aggregates stability, decrease in bigger aggregates, deterioration of soil porosity, increase of bulk density and lowering of bigger pore content. For natural Udic ferralisols, the increase of bulk density and decrease in bigger pore aggregate content were more obvious, however, for cultivated lands, the lowering of aggregates stability and sandification of surface soil were more serious. Mathematical analytic results indicated that the nutrients improvement, especially the organic matter deficient, might be the main interior factor causing structure degradation of Udic ferralisols.