The invention relates to modified coal ash used for treating ammonia nitrogen waste water, which is prepared by the following steps of: calcining NaOH and coal ash at a low temperature after NaOH and coal ash are uniformly mixed in proportion, cooling, grinding and sieving, then adding 1-5 times of water by weight, mixing, stirring, heating to the temperature of 70 DEG C and stopping stirring, gelling for 1.5-2 hours at constant temperature, then heating to the temperature of 100 DEG C, crystallizing for 3-4 hours at constant temperature, carrying out suction filtering, washing to be neutral with water, drying, cooling and then grinding and sieving, thus the modified coal ash is obtained. In the invention, NaOH low temperature calcining hydrothermal synthesis is adopted to modify the coal ash, the physical structure of the coal ash is destroyed, the modification time is shortened and a new zeolite mineral phase is generated under the condition of low alkali addition amount, the specific area of the coal ash is increased, and the cation exchange performance of the coal ash is improved. The modified coal ash provided by the invention is used for treating the ammonia nitrogen waste water, and the removing effect is obvious. By adopting the modified coal ash provided by the invention, wastes are treated with wastes, the resource utilization of wastes is realized, and the utilization factor of the coal ash is improved.