A mathematical model of conventional design for the multi-effect two stage evaporation system with condensed water flash and solid separation was established. The equilibrium was correlated to equation and the model could be solved with computer easily. Then the complex manual calculating was avoided. Mass balance and energy balance equations of the system were expressed in matrix equation that has the advantage of clear structure and high modularization. By holding or omitting correlative block-smatrix in matrix equation, matrix equation could be used to describe the multi-effect two stage evaporation system without condensed water flash or solid separation. The model was solved by combining matrix method with iteration method. The practical example indicated that condensed water flash is an effective method to save energy. And the algorithm increases the convergence speed and stability.