Derivatives of well-bore and grid-block pressure to grid porosity are derived from initial boundary value problems of fluid equations in porous media and inner boundary condition.The derivatives are computed with numerical solution of 3D fluid equation in porous media.A inverse iterative equation is constructed by conjugate method.Numerical inversion of porosity distribution are realized for uniform or block uniform distribution reservoirs.It shows that the relative error of inverse porosity is less than three ten thousandths for a homogenous porosity reservoir with about 8-10 iterations.Relative error of inverse porosity is less than one thousandth for a block homogenous porosity reservoir.Computed porosity automatically approachs to the real value in a few iterations.