Based on the theory of rock mechanics, the deformation mechanismof rock was studied. The porosity of rock depends on the arrangements and the size distribution of grains, not the size of grains. The porosity curve can be divided into the compaction portion and the compression portion. Porosity in the compaction portion changes with stress, while the porosity in compression portion keeps constant and does not change with stress. However the theory of petroleum geology takes the compression portion as the undercompaction portion, and the constant porosity as the result of increased pore pressure. The equivalent depth method is used to calculate the pore pressure. Actually the equivalent depth method is not equivalent. The dependence of porosity on the pressure is lack of theoretical basis. The abnormal pressures in the compression portion by the equivalent depth method are not consistent with practice. Prediction of pressure by the equivalent depth method is meaningless in practice which maymislead the relative researches.