Terzaghis effective stress principle was introduced to the study of the mechanical properties of saturated concrete, and the influence mechanism of water pressure in pore on the strength of concrete under complicated stress states was investigated. The expression of effective stress principle applicable to saturated concrete was established. The theoretical equation for the relationship between pore pressure increment and principal stress increment under general three directional stress condition is proposed. By this model, the compressive strength of concrete under different confining pressures is predicted, and the variation of the matrixs true stress loading path between the saturated and dry concrete was quantitatively described in the compressive meridian planes of the principal stress space. Predicted trends are in good agreement with the experimental results. The results indicate that the existence of water pressure in pore changes the loading path of the matrixs true stress in the principal stress space. Therefore, the compressive strength when the stress state of the concrete reaches its limit is enhanced. The compressive strengths of saturated concrete in different stress states under quasi static loading cases are decreased with the dry samples, which is influenced by the initial hydrostatic stress, pore ratio and loading path. The influence mechanism of the water pressure in pore on the strength of concrete can be interpreted from the viewpoint of matrixs true stress.