Abstract: A forecasting method is proposed for the high rock slope stability of Longtan hydropower station, based on the matter element model as deduced from the theories of extenics. The elements in classical and sectorized field are defined using the stability classification and influencing factors on rock slope, then an extension forecasting model is developed for comprehensive evaluation of the classification of rock slope stability by the correlation function of matter elements and extension sets. The forecasting results are thus obtained through such an evaluation. An exemplification shows that the method proposed can forecast the classification of rock slope quality more accurately and scientifically, thus providing a more reliable basis for taking economic and reasonable supporting/reinforcing measures.