The invention discloses a preparation method of an inorganic-organic composite polymer water treatment defluorination flocculant, which is characterized by comprising the following steps of (1) stirring a sodium silicate solution at a constant temperature to obtain a colorless transparent solution, then adding diluted hydrochloric acid for acidification, and stirring to obtain a solution A, (2) dissolving aluminum chloride hexahydrate and ferric chloride hexahydrate in distilled water, adding the solution A while stirring, then adding sodium carbonate, and stirring to obtain a solution B, (3) dissolving modified chitosan, poly dimethyl diallyl ammonium chloride and sodium hydrogen sulfite in distilled water, adjusting the pH value with diluted hydrochloric acid, and stirring to obtain a solution C, and (4) adding the solution B into the solution C, and stirring for reaction to prepare the defluorination flocculant solution. When the flocculant disclosed by the invention is used for treating the fluorine-containing wastewater, the fluorine content in the effluent can reach the class III first-grade discharge standard of national Environmental Quality Standard for Surface Water (GB3838-2002), the fluorine removal rate is high, the fluorine removal effect is obvious, and deep treatment of the fluorine-containing wastewater can be realized.