NOVELTY - Preparation of coagulant aid for blue algae biogas liquid pretreatment comprises adding acrylamide (AM) solution and reducing agent solution in reaction bottle, filling with nitrogen, adding a fuel gas and an oxidant, reacting for 16 hours to obtain star polyacrylamide (SPAM) colloid; putting the SPAM colloid to sodium hypochlorite (NaClO) and sodium hydroxide (NaOH) mixed solution, reacting via Hoffman rearrangement to obtain SPAM; and adding SPAM Hofmann rearrangement product to starch solution.    USE - A preparing method of coagulant aid for blue algae biogas liquid pre-treatment.    ADVANTAGE - The prepared coagulant aid has extended conformation, so it can obtain common linear high molecular flocculant, better flocculation effect, little dosage, wide pH range, convenient operation and use, and is safe and non-toxic.    DETAILED DESCRIPTION - Preparation of coagulant aid for blue algae biogas liquid pretreatment comprises:    (A) taking acrylamide monomer, reducing agent and oxidant pentaerythritol ammonium ceric nitrate and preparing into 5-10 mass% aqueous solution, in the reaction bottle, adding AM solution and reducing agent solution, filling with nitrogen after fully removing oxygen, adding a fuel gas and an oxidant, uniformly mixing, then placing the reacting bottle in 47.5 degrees C constant temperature water bath, reacting for 16 hours to obtain star polyacrylamide (SPAM) colloid;    (B) putting the SPAM colloid to NaClO and NaOH mixed solution, reacting at 35 degrees C for 20 minutes via Hoffman rearrangement to obtain SPAM;    (C) putting soluble starch and distilled water at a ratio of 1/20 (g/mL) in a container, under the nitrogen condition, heating in a constant-temperature water bath boiler for 90 minutes to 80 degrees C, and then cooling to 48 degrees C; and    (D) adding SPAM Hofmann rearrangement product to the starch solution, reacting for 40 minutes at 48 degrees C, and then using 4 mol/L sulfuric acid solution to the reaction medium which is composed of alkaline-acid pH=6-7, reacting for 20 minutes, stopping the reaction with anhydrous sodium sulfite, using 6 mol/L hydrochloric acid, pH=3-4, acidified into the volume of 10 times of alcohol, separating out the white precipitate, washing with acetone, vacuum drying at 60 degrees C to obtain crude product, washing with mixture of ethylene glycol and glacial acetic acid at a volume ratio of 6:4 for three times, drying at 60 degrees C to obtain the coagulant agent starch grafted-SPAM (ST-g-SPAM) powder products.