In order to reduce the harm of commonly used chain transfer agent during the preparation of zwitterionic polymer viscosity reducer，using acrylic acid （AA），2-acrylamido-2-methylpropanesulfonic acid （AMPS），and methacryloxyethyl trimethylammonium chloride（DMC）as monomers，environmentally friendly tetramethylthiuram disulfide（TMTD）instead of sulfhydryl compound as chain transfer agent，a zwitterionic polymer viscosity reducer with low molecular weight for drilling fluid was synthesized through the aqueous solution explosion polymerization process. Taking the viscosity reduction rate at room temperature as the evaluation index，the synthesis condition of the viscosity reducer was optimized，and then its viscosity reduction performance was evaluated. The results showed that，the optimum synthesis condition was obtained as follows：0.75% TMTD，3% oxidation-reduction initiator in total mass of monomer，50% NaOH in total molar amount of AA and AMPS，44% AA，5.5% AMPS and 4.5% DMC. Under this condition，the number average molecular weight of the synthesized zwitterionic polymer viscosity reducer was 914 g/mol. Its viscosity reduction rate in fresh water base slurry was 91.53%. The viscosity reducer had good temperature resistance，which met the temperature resistance requirement of 180 ℃ . The preparation method reduced the environmental pollution caused by the synthesis process of zwitterionic viscosity reducer with low molecular weight.