The two-scale continuum model has gradually developed into the main mathematical model for the numerical simulation of the acid stimulation process due to its high precision modeling physical process and cheap calculation cost. This chapter focuses on improving the classic two-scale continuum model for acid stimulation. The mass conservation equation of acid fluid is re-derived from the perspective of component mass conservation, considering the mass exchange between solid matrix and acid fluid. The Naiver-Stokes-Darcy equation is used to replace the Darcy equation, considering the interactions between acid fluids and the effects of porous media structure on fluid momentum transport in the matrix acidizing process. The pore-scale model is improved to capture the changes in rock properties and acid component diffusion transport that better fit the actual physical processes. Besides, this chapter also proposes the velocity-pressure double-decoupled marker and cell (MAC) algorithm to numerically discretize the mass conservation equation and the momentum conservation equation for acid fluid on staggered grids. The theoretical, analytical solutions of the steady-state convection-diffusion-reaction equations and the numerical simulation results of the classical acid stimulation have been used to validate the two-scale continuum model.