NOVELTY - Humic acid is prepared by pickling weathered coal with phosphoric acid solution, then filtering, washing filter residue, adding water, then adding 30% ammonia water, refluxing and reacting at 60 degrees C for 2-6 hours, after finishing reaction, filtering to obtain humic acid ammonium solution; adding hydrogen peroxide to increase the phenolic hydroxyl group of humic acid ammonium solution; and adjusting pH of reaction system with to more than 8, and reacting at 30-120 degrees C for 4-10 hours to obtain humic acid having phenolic hydroxyl content of more than 2 mmol/g humic acid solution.    USE - Method of preparing humic acid (claimed) used as fertilizer.    ADVANTAGE - The method utilizes easily available raw material, has low cost, simple technology, no toxicity, and no pollution. The obtained product is stable and has lower cost of 10 yuan/unit as compared to Henan weathered coal yellow humic acid (360 yuan/kg).    DETAILED DESCRIPTION - Preparation of humic acid comprises:    (A) preparing humic acid ammonium solution by washing (pts.wt.) weathered coal (150) with 3 mass% phosphoric acid solution, then filtering, washing filter residue, adding water (450), then adding 30% ammonia water (10-70), refluxing and reacting at 60 degrees C for 2-6 hours in a flask, adjusting final pH to 9-10 with 30% ammonia water or 10% phosphoric acid solution, keeping normal pressure, after finishing reaction, filtering to obtain the filtrate that is the humic acid ammonium solution;    (B) dropping (pts.wt.) antifoaming agent (0.1) into humic acid ammonium solution, then adding copper-iron composite catalyst (0.15-1.5) and EDTA or citric acid (10), and adding 30% hydrogen peroxide (10-80) to increase phenolic hydroxyl group in the humic acid ammonium solution; and    (C) adjusting pH of reaction system with 10% phosphoric acid solution or 30% ammonia water to more than 8, keeping the pressure to 2.9-5 kg/m2, and reacting at 30-120 degrees C for 4-10 hours to obtain humic acid having phenolic hydroxyl content of more than 2 mmol/g humic acid solution.