For engineering applications of water dilution controlling system, the fluid dynamics of a mixed flow was studied with computational fluid dynamics (CFD) simulations and self-designed experimental set-up. In order to examine the predictability of CFD model for the headbox in industrial scale, two pulp suspensions before mixing were treated as homogeneous flows separately. Standard k-epsilon turbulence models with the mass diffusion in turbulent flows-species transport approach were applied in the simulations. A numerical simulation of this headbox model was analyzed with semi-implicit method for pressure linked equations scheme with pressure-velocity coupling. Results show that the model can predict hydrodynamic characteristics of headbox with injecting dilution water in a central diffusion tube, and the distribution of water content at the outlet of the slice lip is ideally normal at different speeds. (C) 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.