This paper analyzed the flow of working medium in extrusion by an athletic differential equation of viscous fluid and discussed the influence of the viscosity on Hot Hydrostatic Extrusion(HHE). Extrusion force of HHE can be minimized and the disfigurement of extrusion products can be avoided when the viscosity of the medium is proper. A mathematics formula for the viscosity of working medium is given important theoretic foundation for HHE.