In the process of concentrating anaerobic fermentation slurry to commercialized liquid organic fertilizer with membrane filtration devices, UF membrane fouling was always a serious problem. The influence of in-line coagulation pretreatment on UF membrane filtration flux was investigated. The experiment applied six inorganic coagulants (ferric trichloride, ferrous sulfate, aluminum sulfate, potassium sulfate, sodium silicate, PAC), and separately studied the best coagulant and its optimal dosage in two groups. Results showed that ferric trichloride was most efficient and relatively less cost. At FeCl3 dosage of 8 g/L, the stable flux was 14.38 times more than the raw water, and only one third of the PAC dosage with the same effect. With the increasing of ferric trichloride dosage, the stable flux increased first and reached to the maximum under the dosage of 10 g/L, then decreased, which is closely related to re-steady phenomenon. Based on an overall consideration of flux improvement, pollutants removal and dosage, FeCl3 dosage at 8 g/L was considered to be the optimal dosage.