The invention discloses a preparation method of humic acid, and aims to solve the problems of high energy consumption, low extraction rate, and loss of a part of biological activity in the traditional method. The method comprises the following steps: A, preparing an extractant, namely a mixture of sodium carbonate, sodium hydroxide and sodium tripolyphosphate; B, adding a co-extraction solvent, wherein the co-extraction agent is a mixture of potassium hydrogen phthalate, ethanol and glycerol; C, adding raw materials, namely slowly adding leonardite raw materials under agitation; D, separating, namely carrying out gravitational settling on a humic acid liquor in a settling pond for 30-40 minutes to carry out solid-liquid separation; E, aging, namely aging the separated humic acid liquor into the tank to age for 10-12 hours, so as to adjust the pH to 5.8-7. The preparation method has the advantages that 1, the energy sources are saved, the efficiency is improved, the extraction time is shortened to 1-1.5 hours, and only 8.6kg of standard coal and 0.74kw.h of power are required in production of each ton of humic acid liquor according with the standard in actual running; 2, no three wastes are discharged in production.