In this study, micro-polluted raw water in winter western mountainous cities was treated by PAC-PDM . The CCD model of response surface methodology was employed to investigate the removal rate of turbidity . Taken the model as constraint conditions and taken the minimum agent cost as the objective function, optimization model of composite coagulants is established. The results showed that when the turbidity was 1 NTU，the dosage of PAC (based on Al2O3) is about 26mg/L, and the proportion of optimization model of composite coagulant PAC-PDM is 5% and the dosage of PAC (based on Al2O3) is about 8mg/L. Therefore the dosage of PAC of the optimized composite coagulant could be decreased by about70%, and agent costs decreased by about 56% compared with those of PAC.