Soil efficient thickness should be analysed in the appraising of erosion risk of soil, In this paper, combined with the charting of soil thickness of QinBa hill in southern Shaanxi, the method and step to chart soil thickness is studied with trend surface analysis and regression analysisy etcc..From the results of charting it can be seen that the mathematical method has nature of high logic, high accuracy of map spot,and that the maps made with this method could reflect the actual distribution and va riation of the soil thickness.