A new volume deformation work conservation equation of saturated porous media was proposed, and the effective stress mechanics mechanism was explored based on deformation work, in view of the validity of Terzaghi effective stress formula in saturated porous media. The mechanical work principle was used to prove the validity of Terzaghi effective stress formula without taking the deformations of solid and fluid matrix into account. The volume strain of solid phase was divided into solid volume fraction strain and solid matrix volume strain based on mixture theory, considering the deformations of solid and fluid matrix. The mechanical relationship of work conjugate between Terzaghi effective stress and solid volume fraction strain was obtained through the derivation and analysis of volume deformation work conservation equation. Terzaghi effective stress can only determine the solid volume fraction strain when the stress mechanisms among solid matrix deformation, solid volume fraction change and fluid matrix deformation were mutually independent under infinitesimal strain condition. The volume deformation of solid phase can be closely determined only if both solid matrix stress and Terzaghi effective stress were used. Results show that Terzaghi effective stress, as one of the internal factors affecting the volume strain of solid phase, does not need to be corrected.