An investigation was carried out on modeling of optimal dosage prediction for coagulation treatment of CSTR fermentation tank effluent of a biogas project. The parameters of optimal coagulant dosage model were determined using regression analysis. Influencing factors of optimal coagulant aid dosage were studied through orthogonal regression experiment, then optimal coagulant aid dosage model was built. Finally a pilot experiment was carried out to verify the reliability of the models. Results showed the model of optimal coagulant dosage in coagulation treatment of biogas slurry Awas 0.00006C1.41; the model of optimal coagulant aid dosage Bwas 0.000148CN-0.0142C-0.277N+26.7, influent chemical oxygen demand and stirring intensity were found to be influencing factors of the coagulant aid PAM dosage. The obtained models were proved accurate and reliable through statistic test and pilot experimental verification.