The invention discloses a method for preparing a flocculating agent based on cesium triiodate visible light initiation. The method comprises the following steps: taking chitosan and hexafluorobutyl methacrylate as raw materials, acrylamide as a cross-linking agent, urea as a cosolvent and triiodoplumbate cesium as an initiator; sequentially adding the three components into a reaction bottle, adding a proper amount of water, and fully and uniformly mixing and completely dissolving the three components under the condition of rapid stirring; adding urea, rapidly stirring, finally adding a cesium triiodo-plumbate 3 initiator, introducing nitrogen to remove oxygen, and uniformly stirring and mixing; finally, reacting the reaction bottle under visible light; and after the reaction is finished and aging is performed, extracting and purifying the product to obtain the modified chitosan-based flocculating agent. The preparation method disclosed by the invention is simple in process, high in production efficiency, relatively low in production cost, less in harm to a human body in an operation process, simple and convenient to operate and easy to control. The obtained modified chitosan-based flocculating agent is relatively good in water solubility, stable in performance and relatively strong in functionality, has a relatively good treatment effect on surface water containing heavy metals, soluble organic matters and the like, and has good social benefits and economic benefits in practical application.