Slope length is an important topographic factor.There are various methods to extract the slope length factor of a watershed. Both of grid based direct calculation method and fast calculation method based on the ridge line have been used in practical research in China. Based on the DEM data of the Danangou wa-tershed in Shaanxi Province, this study attempts to make a comparison of precision for the two methods, and analyze the problems that may exist. Results show that direct calculation method has the eligible ratio of 86.9% and model efficiency of 0.520, whereas fast calculation method has the eligible ratio of 69.7% and model efficiency of-0.150. It can be seen that direct calculation method is superior to fast calculation method. This study can serve the further research work on slope length calculation and other related fields.