Abstract: Based on Baihetan Hydropower Project on Jinsha River, 3D discrete element models for columnar jointed rock mass are established considering the relevant methods of rock mechanics. Numerical simulations of triaxial compression tests on the columnar jointed rock mass are carried out. The impacts of diameter change on the equivalent elastic modulus of hexagonal columnar jointed rock mass as well as those of side length and joint stagger change on the equivalent elastic modulus of quadrangular columnar jointed rock mass are studied. In order to investigate the impacts of size change on the equivalent elastic modulus of random columnar jointed rock mass, a program for controlling columnar size is proposed using the average large diagonal size. Then the impacts of change of the average large diagonal size on the equivalent elastic modulus are studied. It is shown from the researches on size effect and comparison with the in-situ tests that changes in cylinder size have larger impacts on the equivalent elastic modulus perpendicular to cylinder than on that parallel to cylinder. Changes in stagger of quadrangular columnar jointed rock mass mainly have impacts on the equivalent elastic modulus parallel to stagger and have weak impacts on the equivalent elastic moduli in two other directions. The results can provide reference for the determination of mechanical parameters in engineering practice.