The invention provides a water quality adjusting method for a waste heat boiler. A treatment mode of equilibrium phosphate is adopted, that is, a mixed solution of sodium hydroxide and phosphate is added into the boiler, and the content of the phosphate in boiler water is enabled to be reduced to the concentration required by reaction of the phosphate and hardware components, the content of NaOH in the mixed solution is 60 to 65 percent, and the content of Na3PO4 is 35 to 40 percent; the boiler water is allowed to contain 0.6 to 0.8mg/L of NaOH, so that a PH value of the boiler water is enabled to be within a qualified range; 0.8 to 1.2kg/m<3> of the mixed solution of the sodium hydroxide and the phosphate is added into the boiler water every 16 to 24 hours, one-time sampling inspection of a PH value and a phosphate radical is performed in each hour, and records are made. The index qualified rate of the boiler water of the waste heat boiler provided by the invention reaches 95 percent or above, thereby ensuring safe and stable operation of the boiler; the suspended production phenomenon because of faults of a coke dry quenching boiler caused by water quality problem is avoided, the steam production amount and the coke mass of the boiler are improved, the cleaning workload of the boiler and expenses of spare parts are reduced, and picking times of the boiler and the consumption of saline water are reduced.