It is essential for regional soil erosion survey and monitoring to calculate LS factor at the watershed and regional scales. Based on the theories of slope hydrology and soil erosion and the technologies of digital terrain analysis,the principles for slope and length factor are summarized and the methods and flowchart are suggested. By taking Xiannangou as an example,the LS themes are extracted and the characters are analyzed statistically and cartographically. The issues that should be studied urgently are discussed at last.