Abstract: Long-term stability of the dam foundation is an important guarantee for the safety of dam operations, and the weak interlayer in the rock mass of dam foundation is an important factor which affects the stability and deformation. In order to study the deformation mechanism of the weak interlayer in diabase dikes under long-term load at Dagangshan hydropower dam foundation, in-situ large-scale compressive creep test using circular rigid bearing plate is carried out which is perpendicular to the weak interlayer in test cavern of dam slope. The five-parameter generalized Kelvin model is identified to describe the creep property of weak interlayer, which avoids the defects of three-parameter generalized Kelvin model that has the problem of unwanted rapid convergence. Based on the Boussinesq problem in elastic mechanics, the visco-elastic deformation formula of five-parameter generalized Kelvin model for rock mass under rigid bearing plate is derived for the first time through Laplace transform and inverse transform, and the rheological parameters are inversed.