The northeast mountain region of Yunnan province is a major region of soil and water conservation of the upper Yangtzeriver in Yunnan province, the chief part of soil erosion in this region is cultivated slopeland. By means of the statistical analysis of the experimental data measured on 32 experimental spots over three years continuo usly, the soil loss equation of cultivated slopeland in northeast mountain region of Yunnan province has been developed as follow s: A = R·K·LS·C·P . The algo rithm and data for every factor in the equation as well as the capacity for permitable loss of soil in northeast mountain region of Yunnan province have been determined, which provided the fundamental technological data for future use of the equation. At the same time, the author has made a test to the equation,the results indicate that the error of data by calculating does not exceed 6% when compared with the practical experimental data, which proved the equation may be very reliable in futureapplication. The development of the equation may provide a series of dependable scientific methods and bases for predicting soil erosion, mapping out the rational utilization planning of land and the countermeasures of soil and water conservation, and carrying out the strategy of sustainable develo pment in northeast mountain region of Yunnan province.