NOVELTY - A low heat value coal preparing method involves using domestic sewage treatment plant sludge, preparing a catalyst, followed by adding nitric acid solution into L683K catalyst and carrying out reaction to obtain a reacted product. The pH of the reacted product is adjusted, followed by adding phosphonium solution and sulfuric acid solution to the reacted product to react, followed by adding sodium hydroxide powder to the product to adjust pH. The resultant product is cooled and ground to obtain powder. The catalyst is mixed with wastewater treatment plant sludge to obtain a mixed product.    USE - Method for preparing low heat value coal.    ADVANTAGE - The method enables preparing low heat value coal with reduced energy conservation and emission in a simple and inexpensive manner.    DETAILED DESCRIPTION - A low heat value coal preparing method involves using domestic sewage treatment plant sludge, preparing a catalyst by mixing 0.6-5 wt.% titanium dioxide, 0.2-5 wt.% oxide, 0.1-5 wt.% celestite powder, 0.7-5 wt.% mullite powder, 0.3-3 wt.% serpentine powder, 0.15-3 wt.% montmorillonite powder, 1.2-3 wt.% granite powder, 0.9-3 wt.% bentonite powder, 0.08-1 wt.% rutile powder, 0.95-8 wt.% mixture comprising petrol and butane at a ratio of 1:1, 3-8 wt.% marble powder, 0.03-1 wt.% cryolite powder, 2-5 wt.% talc flour, 0.001-1 wt.% gangue powder, 5.2-7 wt.% sodium carbonate, 1.6-5 wt.% Kaolin powder, 0.4-5 wt.% sepiolite powder, 3-5 wt.% iron powder, 3.9-5 wt.% quartz powder, 2.2-5 wt.% diatomaceous earth powder, 0.8-3 wt.% bauxite powder, 2-3 wt.% attapulgite powder, 0.37-3 wt.% rock phosphate, 0.15-0.5 wt.% graphite powder, 0.88-1 wt.% mixed rare earth waste, 0.039-1 wt.% salt-containing polymer lithium silicate, 0.5-1 wt.% potassium, 0.83-1 wt.% magnesite chloride and 1-5 wt.% clay powder, followed by adding nitric acid solution into L683K catalyst and carrying out reaction at 80-120 degrees C to obtain reacted product. The pH of the reacted product is adjusted to 1-2, followed by adding phosphonium solution to the reacted product to carry out reaction at 100-120 degrees C. The pH of the reacted product is adjusted to 1-2, followed by adding sulfuric acid solution to the product to react at 120-150 degrees C. The pH of the reacted product is adjusted to 1-2, followed by adding sodium hydroxide powder to the product to adjust pH of 5.5-14. The resultant product is cooled and ground to obtain 80-120 mesh powder. The catalyst is mixed with wastewater treatment plant sludge with water content of 50-85% at a weight ratio of 100:0.05-40 to obtain mixed product. The mixed product is placed in reaction kettle at more than 0 degrees C for 200-240 hours. The temperature is raised to 320 degrees C and 180 degrees C at a speed of 1 degrees C, followed by introducing nitrogen from 0.1-10 MP a 20-24 hours, and maintaining pressure till the temperature to be reached to 240 degrees C. The kettle is pressurized to 15 MP by passing 50-60% oxygen and 30-50% nitrogen mixed gas for 60-78 hours, and to 20 MP by passing 70-90% hydrogen and 10-30% nitrogen mixed gas for 15-20 hours. The pressure is maintained to carry out heating process at 320 degrees C. The temperature is raised to 580 degrees C by raising 1 degrees C for every 50-100 hours. The temperature is maintained for 10-20 hours. The resultant product is cooled to 400-420 degrees C. The kettle is pressurized by passing nitrogen gas to 150-160MP for 10-60 hours and cooled naturally to obtain a finished product. An INDEPENDENT CLAIM is also included for a catalyst.