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Algae flocculating agent is nontoxic and easy to be degraded, which can be used to pre-treat biogas slurry. Based on the existing literature and research, four kinds of algae (Porphyridium cruentum, Chlamydomonas reinhardtii, Scenedesmus obliqnus and Chlorella sp.) were cultured under the same conditions for 15d and the growth period were determined every day. Meanwhile, culture medium was collected to flocculate the biogas slurry every day. Flocculating rate was measured by optical density value. The highest flocculating rate of those four species algae in 15d is as following: the flocculation rate of Porphyridium cruentum reached 39.01% on the 7th day, Chlorella sp. reached 26.99% on the 11th day, Chlamydomonas reinhardtii reached 25.93% on the 8th day and Scenedesmus obliquus reached 25.39% on the 6th day. Considering the flocculation effect, Porphyridium cruentum was better than the other algae in several continuous days. Then Porphyridium cruentum was cultured under its best optimal culture conditions: the light intensity of 3000lx, the temperature of 30℃, light ratio of 12h〖DK〗∶12h, inoculation amount of 30%, pH value of 7 and salinity of 3%. The culture medium was collected after 7d. Based on center composite test, four factors were chosen, including flocculation concentration, flocculation reaction time, temperature of flocculation and initial pH value. The flocculation rate reached the highest of 76.25%, and the corresponding optimum conditions were as following: pH value of 7.69, temperature of 26.78℃, flocculation concentration of 47.70mL and flocculation reaction time of 20.39min. The parameters of the corresponding flocculated biogas slurry were measured, including the corresponding removal rate of TN, ammonia nitrogen, TP, TS, VS, SS and COD were 51.30%, 54.67%, 57.00%, 54.11%, 57.80%, 53.71% and 43.16%, respectively. Under the condition of optimizing flocculation parameters, the flocculating substances produced by Porphyridium cruentum had better removal effect on various kinds of nutrient in biogas slurry.