P (AMDAC) is synthesized using acrylamide (AM) and acryloxyethyltrimethyl ammonium chloride (DAC) by photopolymerization technique in aqueous solution and P (AMDAC) in different molecular weight to study the influence on the efficiency of municipal sludge dewatering. The study results show that the optimum synthesis conditions are photoinitiator mass ratio of 0.50%, monomer mass ratio of 30%, solubilizer mass ratio of 0.40%, pH value of 5.0. Under optimum conditions, the molecular weight is 1.02×107, and with coagulant dosage of 0.5 g·kg1, the filter cake moisture content and filtrate residual turbidity are 65.9% and 4.52 NTU respectively.