The invention discloses a filtrate reducer for a soil-free high-density saturated brine drilling fluid. The filtrate reducer is a mixture formed by a polymer A and a polymer B and with an interpenetrating copolymer network, wherein the polymer A is composed of starch and an ethylenic monomer by polymerization, and the polymer B is formed by an ethylenic monomer and a silicone monomer containing double bonds by polymerization. In addition, the preparation method is simple, the preparation method is simple, a mixed solution with the polymer A and a mixed solution with the polymer B obtained by prepolymerization are mixed and stirred to form the interpenetrating copolymer network, and then the solution is subjected to ethanol precipitation, washing, drying and pulverizing to obtain the reducer. The filtrate reducer is moderate, the filtrate loss reduction effect is remarkable, the temperature and salt resistance is good, and the solid phase has a good suspension effect.