Viscosity reducer for water-based drilling fluid as well as preparation method and application of viscosity reducer

The invention discloses a viscosity reducer for water-based drilling fluid as well as a preparation method and application of the viscosity reducer. The viscosity reducer for the water-based drilling fluid is obtained by carrying out copolymerization reaction on a carboxyl group-containing monomer A, a sulfonic group-containing monomer B and a cationic monomer C in water, adjusting the pH value to 8-9 after the copolymerization reaction, drying and crushing, and has the following structural general formula: # imgabs0. The viscosity reducer for the water-based drilling fluid can resist the temperature of 260 DEG C or above; compared with an existing drilling fluid viscosity reducer, the viscosity reducer has the advantages that the viscosity reduction performance can be improved by about 20% under the conditions of high temperature and salt and calcium pollution, the rheological property of the drilling fluid is effectively improved, and the viscosity reducer is suitable for drilling of high-temperature stratu