Abstract: With the gradual deepening of coal mining layer in China, fundamental understanding of mechanical characteristics and constitutive relationship of the coal-rock combined body is increasingly significant. The original crack and interlayer between coal and rock have a major influence on the mechanical properties of the coal-rock combined body. In order to investigate the influence of crack and interlayer, the evolution characteristics of axial crack strain along with axial strain and axial stress are studied, and based on the concept of engineering strain and natural (true) strain, the Axial Crack Closure Model (ACCM) and the Axial Crack Propagation Model (ACPM) are established. Finally the constitutive model for pre-peak stress-strain curve is proposed. It is found that on the basis of the axial crack strain changing along with the axial strain, there are 4 stages in the whole process: initial stage, stable stage, slow-growth stage and rapid-growth stage. The characteristics of the axial crack strain evolving with the axial stress are similar to those changing with the axial strain. The ACCM, ACPM and axial stress-strain model are found to fit with the experimental data well, and they can describe the nonlinear characteristic of stress-strain relationship.