Copolymer Filtration Reducer is polymerized with solvent method through the optimization of monomer of salt resistance and high temperature resistance. The steric hindrance and polymer molecular rigidity are enhanced because of reasonable space volume of the monomers. So Copolymer Filtration Reducer overcome the disadvantages of poor shear performance, poor salt resistance, poor FLHTHP performance of conventional polymer filtration reducer, is a new polymer filtration reducer with exnt performance. It can be applied in deep wells, ultra-deep wells and high-salinity brine drilling fluids. The recommend dosage is 3.0 - 7.0ppb.