The invention discloses a polymeric titanium sulfate-PDMDAAC (poly dimethyl diallyl ammonium chloride) composite coagulant as well as preparation and application methods thereof. The preparation method comprises the following steps: 1, preparing a titanium sulfate solution with certain concentration by adding water, placing the titanium sulfate solution in a mixing kettle, dropwise adding a sodium hydroxide solution by controlling certain speed, performing polymerization reaction, stopping adding the materials when the basicity (B) is 0.5-2.5, and continuously stirring for 0.5-1.5h after finishing adding, so as to prepare a colorless and transparent polymeric titanium sulfate solution; 2, placing the prepared polymeric titanium sulfate solution in a mixing reaction kettle, matching according to the fact that the mass ratio of Ti to PDMDAAC is (4-10):1, adding branched-chain PDMDAAC colloid or dry powder under the normal temperature or heating condition; 3, stirring the mixed solution at the normal temperature for 2-4 h, and filtering away insoluble substances, so as to obtain the polymeric titanium sulfate-PDMDAAC inorganic composite coagulant and polymeric titanium sulfate-PDMDAAC organic composite coagulant. The polymeric titanium sulfate-PDMDAAC composite coagulant can be widely applied to fields such as water supply, waste water treatment, papermaking, textile dyeing and printing, and daily-use chemical industry, and has good water treatment effect.