Abstract: This paper argues against the know ledge that hydrostatic pressure is equal to the gravity value of the overlying rocks in studying a dynamic state of certain underground site, and discusses the influence of tectonic stress on hydrostatic pressure and its application. The research of solid mechanics and data from deep drillings indicated that gravity and tectonic stress in middle and shallow crust are directional external forces, whereas hydrostatic pressure is a force of non-directionless, so they possess different physical meanings. Gravity is not quantitatively equal to hydrostatic pressure either. During 10^8 a –10^6 a of stress are laxation, the directional force on solid rock of the crust produces a deviatoric stress field. The authors suggest that stress field T of the crust is a combination or superposition of hydrostatic pressure P with differential stress , and the total hydrostatic pressure P at any point in the crust comprises two parts, one is spherical stress tensor PR caused by the gravity and the other is spherical stress tensor P caused by tectonic stress, therefore P is not only from the gravity of overlying rocks. The results obtained by a finite element simulation indicate that the additiona1 hydrostatic pressures borne by rocks decrease gradually from the compressive zone (Psc) to the shear zone (Pssh)and to the tensile zone (Pst), i.e. Psc> Pssh >Pst; and the difference of the tectonic additional hydrostatic pressure between these deform ed zones trends to increase, following the increase in absolute value and or difference of external forces in different directions. So the tectonic additional hydrostatic pressure is a major factor causing the heterogeneity of 1ocal stress field in the same depth of the crust. The above mentioned research has a wide range of application: 1) to set up a mode1 of unequal hydrostatic pressures in same depth of the crust; 2) to establish the method of measurement of metallogenetic depth corrected by structure, i.e., firstly minute tectonic added hydrostatic pressure P from general hydrostatic pressure P, then measure and calculate the depth data of overlying rocks; 3) to get into a Flew research field of tectonic physicochemistry; 4) reconsider the depth of eclogite in superhtgh pressure metamorphic zone,. such as the knowledge that the diamond and coesite in Dabie tectonic-metamorphic zone are probably the products of the inner crust; and 5) to introduce some new ideas about experiments of chemical kinetics.