A numerical method called the block-spring model for analyzing heavily jointed rocks is presented in this paper. This model permits the assessment of stability of both underground and surface excavation in jointed rock mass and is capable of simulating large displacements. The model simulates the jointed rock mass by an assemblage of rigid blocks interacting through contacts. By relating the contact forces to the relative displacements between the blocks and directly applying the equilibrium equations, the displacements of the blocks and subsequently the contact forces can be determined. An iteration procedure is applied to describe the progressive failure along the joints. The proposed model can identify unstable blocks by considering the rearrangement of the blocks. A computer program has been written inFORTRANbased on the procedures of the proposed model. Some simple examples are presented in the present paper to demonstrate the capability of the proposed model. Advanced features of the model including simulation of rock bolts and groundwater are presented in a companion paper, in which two case histories have been analyzed in detail with the proposed model.Key words: jointed rocks, stress, deformation, analysis, numerical model, excavation, anchors, groundwater.