The invention relates to a preparation method of a water-soluble agricultural soil loosening agent. The method includes the following steps: respectively preparing a monomer mixture A and a mixture solution B; adding the mixture solution B to a reaction kettle equipped with the monomer mixture A, adding ammonia water and a hydrolysis agent, carrying out a neutralization reaction at a controlled temperature of 30DEG C or less to obtain a mixture C; and adding a cross-linking agent and an initiator, and heating above materials to carry out a reaction in order to obtain the soil loosening agent product. Polyacrylamide in the soil loosening agent obtained in the invention has a small molecular weight of 2,000,000-5,000,000, and has large hydrolysis resistance and strong flocculating ability; compared with traditional methods, the preparation method has the advantages of simplicity, easiness in operation, low reaction temperature and mild reaction process; and the product has the advantages of uniformity, no need of drying, great reduction of the production cost, and very good application prospect in liquid water-soluble fertilizers and compound fertilizers.