Long-term strength is one of important properties to predict the long-term stability for rock engineering. New methods for determining the long-term strength that could be called limit strain method and inflection point method were proposed based on the isostrain-rate creep curve. To verify these methods, the traditional tests and muti-stage creep tests were carried out to analyze the creep behavior of greenschist in Jinping Ⅱ hydropower station. On the basis of the tests and proposed methods, the long-term strength under different confining pressure were obtained. Combining with the creep curves, the value of limit strain and stability time of deformation were also obtained in the solving process of long-term strength. The research results show that: the long-term strength can be determined by these two methods and the results meet the empirical value. The ratio of long-term strength and instantaneous strength were 66.9% (uniaxial), 68.0% (confining pressure 10 MPa), 79.6% (confining pressure 40 MPa). Therefore, the two proposed methods are reasonable and the ratio of long-term strength and instantaneous strength increases with the increasing of the confining pressure, and the confining pressure is one of the important aspects that influence the long-term strength.