According to a vacuum phase change wastewater concentration and flue gas waste heat recovery system provided by the invention, flue gas exchanges heat with a first heat exchange medium after passing through a coal economizer, and a flash tank realizes multistage flash evaporation by utilizing gradient vacuum, so that the concentration of desulfurization wastewater is facilitated; meanwhile, steamwith different temperatures can be formed through multi-stage flash evaporation, the steam and a second heat exchange medium are subjected to stepped heat exchange after entering the heat exchange assembly, and the heat exchange effect is improved; due to the economizer, the flue gas waste heat is used for increasing the wastewater temperature; after wastewater flash evaporation, recovered flue gas waste heat is brought out along with steam, the second heat exchange medium recovers heat contained in the steam, and finally the heat is returned to a low-pressure heater system, so that gradient utilization of heat energy is realized under the condition that the flue gas waste heat is hardly lost, the defect that high-quality heat energy is consumed by utilizing a conventional route is overcome, and the energy consumption of a unit is reduced. By means of the mode, waste water concentration is achieved, meanwhile, the problem of flue gas waste heat recovery of a coal-fired power plant or other industries is solved, and good social and economic influences are achieved.