Abstract: In the process of conventional purification of discharged residual guar fracturing fluid, only residue content is required to meet applicable discharging standard, but no consideration has been given to granular diameters of such residues after purification. Taking the residual grain diameter of no more than 1 μm as the benchmark for assessment, “flocculation+active carbon” treatment was used for purification of discharged fracturing fluid. Test results show three conventional flocculants: alum, polymeric aluminum chloride (PAC) and polymeric aluminum-ferric chloride sulfate (PAFCS) can be used to produce residues with grain diameters of 1~2 μm. Among them, PAFCS displays the best flocculation performance at optimal test conditions as follows, Volume of agent is 2.5 g/L, grain diameter is 0.25~0.3 mm, stirring time is 20 min, settling time is 30 min, active carbon with concentration of 4.5 g/L, 4 g/L and 3.5 g/L, respectively, were added to the fluid produced upon completion of flocculation involving alum, PAC and PAFCS. After completion of purification, all residues have grain diameters less than 1 μm, and both grain diameter and content of residues conform to applicable discharging standard.