With the continuous petroleum exploration around the world, the target of exploration is converting from conventional to deep, unconventional reservoirs. The pressure condition on those new target layers is different from the conventional reservoir, and pressure is one of the important factors affecting the elastic moduli and velocities of shale reservoirs. Therefore, it is necessary to take pressure effects into consideration in rock-physics modeling. We initially adopted a novel iterative rock-physical modeling approach on the basis of pore space stiffness theory to analyze pressure’s influence on shale. Pore space stiffness theory assumes that pores own their own stiffness like the solid matrix of rocks, which is related to the effective pressure. We have implemented plenty of numerical analysis on the effect of pressure on shale and found out the most contributive factor to the moduli and velocities of shale. Our result indicates that when the effective pressure is smaller than the critical value, the elastic moduli and elastic wave velocities of shale increase significantly with the increase of effective pressure. The elastic moduli and velocities tend to be constant when the effective pressure goes beyond that critical value. By comparison, we found that the most contributive mineral in shale is clay, and the porosity has the greatest effect on elastic moduli and velocities when the mineral composition of shale stays unchanged. The influence of pressure is not as obvious as other factors in shale reservoirs with low porosity (lower than 10%) because pores occupy a relatively small percentage of the total volume.