This paper presents a method for calculating the macroscopic properties of matrix composites with nonlinear viscous components. A distinctive feature of the method is calculating the coefficients of concentration of average rates of creep deformation by effective volumes of averaging phases. Effective volumes of averaging phases are found by solving the boundary value problem of viscous deformation of the simplified structural model of a two-phase composite. This takes into consideration the ultimate version of the conditionally porous composite with zero material constants of switching. A good agreement between calculation results and experimental data of describing the melt flow of polymer blends with high viscosity inclusions was obtained