Abstract: The determination of rock mass deformation modulus is a difficult problem in rock mass field.Yujianhe water conservancy project is a important project.Rock mass deformation modulus on dam foundation will affect the design of the arc dam.Based on grouping of dam foundation rock mass,rock mass deformation modulus are calculated by RMR and fractional model of rock mass damage.The propositional rock mass deformation modulus is given with the original examination and analogy of engineering geology to provide the seasonal parameters to design the arc dam.