NOVELTY - The method comprises: carrying out biological treatment for coal gasification wastewater; performing desalination process for the wastewater to remove salt for obtaining salt brine and purified water; concentrating the salt brine and purified water in a brine processing unit comprising a lime clarification tank, a multi-medium filter, an ultra-filtration system, a nano-filtration system and a reverse osmosis system to obtain concentrated brine and concentrated water; and then evaporating the concentrated mixture for purifying the water.    USE - The method is useful for performing zero-discharge coal gasification wastewater treatment (claimed).    ADVANTAGE - The method is capable of processing wastewater completely so as to save water resource.    DETAILED DESCRIPTION - The method comprises: carrying out biological treatment for coal gasification wastewater; performing desalination process for the wastewater to remove salt for obtaining salt brine and purified water; concentrating the salt brine and purified water in a brine processing unit comprising a lime clarification tank, a multi-medium filter, an ultra-filtration system, a nano-filtration system and a reverse osmosis system to obtain concentrated brine and concentrated water; and then evaporating the concentrated mixture for purifying the water. The method further comprises carrying out pre-processing of the coal gasification wastewater in a pre-processing unit comprising an adjusting pool that is provided with a stirring device, where hydraulic retention time of the adjusting pool is 12-24 hours. The biological treatment step is carried out in a biochemical treatment unit comprising an anaerobic tank, an anoxic tank, an aerobic tank and a settling tank, where the water is treated in the anaerobic tank for 12-24 hours, in the aerobic tank for 40-72 hours, and in the sedimentation tank for 4-8 hours. The concentration of the sludge in the aerobic tank is 3-4 g/l, dissolved oxygen concentration in the aerobic tank is 2-3 mg/l, and the reflux ratio of the sludge in the aerobic tank is 100-200%. The desalination step is carried out in a salt-removing unit connected to a coagulating sedimentation tank, an ozone oxidation reactor, an aeration biological filter tank, an immersion-type ultra-filtration system and an anti-pollution reverse osmosis membrane. The desalination step is carried out by adding 350-400 mg/l of coagulant in the coagulating sedimentation tank, adding 80-120 mg/l of ozone in the ozone oxidation reactor, adding filter material in the aeration biological filter tank, performing filtration in the ultra-filtration system at filtering flux of 20-40 l/m2.h, and performing reverse osmosis process in the osmosis membrane having recovery rate of 65-75%. The hydraulic retention time of the coagulating sedimentation tank is 3.5-6 hours. The wastewater is treated in the ozone oxidation reactor for 1-2 hours and treated in the aeration biological filter tank for 6-8 hours. The concentration step is carried out by adding 100-300 mg/l of quicklime and 150-250 mg/l of ferric sulfate in the brine processing unit and carrying out filtration at speed of 8-15 m/h and filtration flux of 25-30 l/m2.h. The recovery rate of the reverse osmosis system in the brine processing unit is 70-80%. The recovery rate of the nano-filtration system in the brine processing unit is 70-80%. The water inlet of the reverse osmosis system has pH value of 9-10. The processing unit further comprises a manganese sand filter that is connected with the multi-medium filter and the ultra-filtration system, where filtration velocity of the filter is 8-10 m/h. The evaporation step is carried out in an evaporation unit comprising an evaporator having a recovery rate of greater than 90%. An INDEPENDENT CLAIM is included for a zero-discharge coal gasification wastewater treatment system.