Abstract: The rheological property of rocks is an important factor affecting the stability of dam foundation. The rheological long-term strength is an important mechanical index for evaluating the long stability of dam foundation. The rheological long-term strength of diabase is analyzed by using the stress-strain isochronous curve method, unsteady creep discriminated method, rheological volume strain method and loading-unloading rheological residual strain method according to the triaxial rheological test results of diabase at the dam area of Dagangshan Hydropower Station. The results show that the four methods are all applicable to the long-term strength assessment of complete rock samples, but the results from the rheological volume strain method are more consistent with the actual situation for the jointed rock samples. The scanning electron microscope observation of rock failure samples shows that the long-term macro strength of diabase is mainly determined by the fastness degree of mosaic combination structure in mineral grain and cementation degree between different minerals.