By using maximum upstream flow path, a self-developed new method for calculating slope length value based on Arc Macro Language (AML), five groups of DEM data for different regions in Bijie Prefecture of Guizhou Province were extracted to compute the slope length and topographical factors in the Prefecture. The time cost for calculating the slope length and the values of the topographical factors were analyzed, and compared with those by iterative slope length method based on AML (ISLA) and on C++ (ISLC). The results showed that the new method was feasible to calculate the slope length and topographical factors in revised universal soil loss model, and had the same effect as iterative slope length method. Comparing with ISLA, the new method had a high computing efficiency and greatly decreased the time consumption, and could be applied to a large area to estimate the slope length and topographical factors based on AML. Comparing with ISLC, the new method had the similar computing efficiency, but its coding was easily to be written, modified, and debugged by using AML. Therefore, the new method could be more broadly used by GIS users.