The invention relates to a flocculation recycling method of a rare earth modified nano-titanium dioxide photo-catalyst. The method includes the steps: adding PAC (poly aluminum chloride) flocculatingagent with high Al13 content into waste water treated by the TiO2 photo-catalyst; continuously stirring mixture, enabling tiny TiO2 particles to flocculate into large flocs to precipitate under flocculation action, and separating the large flocs and water; performing standing, filtering and washing to obtain a titanium dioxide powder photo-catalyst containing few impurities. According to the flocculation recycling method, the poly aluminum chloride with high Al13 content serves as a flocculating agent, the rare earth modified nano-titanium dioxide photo-catalyst is recycled by a flocculating method, the method has the advantages of high recycling rate of the photo-catalyst, less of flocculating agent consumption, simple and efficient process, good secondary photo-catalysis activity, environmental protection and the like, the titanium dioxide photo-catalyst is economically, rapidly and efficiently separated, and the flocculation recycling method has a good application prospect for the titanium dioxide photo-catalyst in the field of photo-catalysis treatment of waste water.