The invention provides a preparation method of an isoprothiolane suspending agent by employing water as a solvent. The method comprises the following steps: heating deionized water to 30-80 DEG C, adding the deionized water to a high-shear kettle, stirring, and adding a dispersing agent until the dispersing agent is completely dissolved; feeding the following raw materials in percentage by weight: 10%-60% of isoprothiolane, 0.1%-10% of a wetting agent, 0.1%-10% of a thickening agent, 0.1%-8% of an antifreezing agent, 0.1%-5% of a defoaming agent, 0.1%-10% of a dispersing agent and the balance being the deionized water to the high-shear kettle, pulping, carrying out high-speed shearing, and cooling; pumping pulp into a sand mill through a pipeline and a diaphragm pump, and grinding at a high speed; and adjusting the particle sizes of suspending agent particles to be 0.1-4 microns, and adjusting the pH to be 6.5, so as to obtain a qualified isoprothiolane suspending agent product. Compared with missible oil, the isoprothiolane suspending agent has the beneficial effects that use of a lot of organic solvents and the problems of combustion and pesticide hazards generated by the organic solvents are avoided; the effects on environment, caused by the organic solvents, are completely eradicated; and compared with wettable powder, the isoprothiolane suspending agent is free of dust flying and drifting, safe to an operator and the environment, low in cost, and safe to produce, store, transport and use.