Abstract: Deformation parameters of engineering rock mass are important necessary precondition for deformation calculation and stability analysis.So the reasonable selection on the deformation parameters is a focus and difficulty in geotechnical engineering.Because of size effect,the mechanical parameters would change with the research size of rock mass.Mechanical parameters from laboratory test,even from in-situ test can't exactly represent the characteristics of the engineering rock mass.In order to get mechanical parameters of the engineering rock mass that can be applied to practical engineering directly,the size effect of rock mass parameters and its mechanism of production need to be systemically studied.The size effect of rock mass deformation parameters is studied by multi-scale methods combined with specific engineering project.Deformation modulus with different sizes of rock mass are got from the experiments of laboratory test,in-situ plate load test,wave velocity test,numerical simulation and displacement back analysis.It is found that the rock mass deformation modulus decreases significantly as the rock mass size increases.When the tested rock mass size is larger than 30m,the deformation modulus of rock mass tends to be stable.Equations between deformation modulus and rock mass size are got with regression analysis.Then the macro-mechanical parameters and the REV scale are obtained.Finally,the mechanism of the size effect to rock mass deformation modulus is discussed.It shows the structural planes with different size within rock mass are the intrinsic cause for the mechanism.The REV scale of jointed rock mass is about 3 times of the length of the main structure.When the size is larger than the REV scale,the deformation modulus of rock mass would became a stable value.It is a new attempt to survey the macro parameters of rock mass by combination of multi-scale methods and statistics theory.The findings presented in this paper can provide a useful reference to the parameter determination of rock mass in geotechnique engineering and other similar projects.