In order to study the compression strength and its changing mechanism of the concrete specimen

the numerical simulation method is applied. By simulating the failure process of concrete with different length/diameter ratios under axial compression

the internal stress state and the damage evolution process of crack are studied. Moreover

the changing mechanism of concrete strength under different confining pressures is also analyzed. The result shows that with the increase of length/diameter ratio

the intensity of the concrete specimen is reduced

and the failure surface of the specimen gradually decreases. The concrete intensity change with the internal stress state; the smaller the confining pressure

the weaker the concrete strength. The fracture mode of specimen also changes gradually from shear failure to tensile fracture. The tensile failure surface with a length/diameter ratio of 1 : 10 is recommended as the ultimate tensile strain in unconfined compression condition.