The invention discloses a filtrate reducer capable of resisting high temperature and high-concentration salt and used for a drilling fluid for oil and gas fields and a preparation method thereof. The filtrate reducer is prepared by subjecting three monomers consisting of N-vinyl pyrrolidone, 2-acrylamido-2-methylpropanesulfonate and N,N-dimethylacrylamide to radical copolymerization under the action of a redox initiator and then subjecting a copolymer obtained after radical copolymerization and a sodium hydroxide solution to neutralization. The filtrate reducer provided by the invention can substantially reduce the amount of filtrate loss and resist a high temperature of 220 DEG C and salinity of saturated salt water, and the high-temperature high-pressure filtration loss amount of the filtrate reducer is 10 ml after hot ageing of the filtrate reducer under the condition of a density of 2.3 SG at 220 DEG C for 16 h.