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　evaporatin　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.