The model of complex countercurrent multi-effect evaporation with condensed water flash and extra vapor withdrawal is established. The conventional model of countercurrent multi-effect evaporation is a simplified example of the new model.The new algorithm which combines matrix method with iteration method is presented.The new method is an effective method for solving the model of multi-effect evaporation system

and increases the convergence speed and stability compared with Boryden method.The practical example indicates that the method used in complex countercurrent multi-effect evaporation system is fast and accurate.The vapor consumption of three-effect evaporation with condensed water flash is decreased by about 10% for condensation of NaOH solution. Loss of heat energy increases for preheating of feed with fresh vapor or extra vapor withdrawal.