The　mathematical　model　of　complex　countercurrent　multi-effect　evaporation　with　condensed　water　flash　is　established,　which　aims　at　minimum　annual　cost　(including　cost　of　fresh　vapor,　vacuum　pump　power,　evaporator,　maintenance　and　depreciation).　Based　on　such　decisive　variables　as　the　pressure　of　fresh　vapor,　the　pressure　of　condenser　and　heat-transfer　mean　temperature　difference　of　vari-effect,　the　model　is　solved　by　the　complex　method,　Lagrangian　multiplier　method,　iteration　method　and　matrix　method.　The　practical　example　indicates　that　the　influence　of　the　pressure　of　fresh　vapor,　the　pressure　of　condenser　and　flash　of　condensed　water　on　the　optimum　design　results　is　notable.　The　annual　cost　calculated　by　the　optimum　method　is　decreased　by　about　11%　against　that　calculated　by　the　conventional　method.　The　new　algorithm　increase　the　convergence　speed　and　stability.