The copolymer, which was used as fluid loss additive for drilling fluid was synthesized by free radical polymerization in aqueous solution, using N-Vinyl-2-pyrrolidon (NVP), Dimethyl- aminoethyl acrylate methylchloride (DAC), 2-Acrylamide-2-methylpropanesulfonic acid (AMPS) and acrylamide (AM) as monomers, ammonium persulfate and sodium sulfite as initiators. The synthetic conditions with optimal property were achieved by the orthogonal experiment, and the structure of copolymer was characterized by FT-IR. The results of performance evaluation showed that the synthesized copolymer had excellent temperature resistance, good heat resistance as well as salt tolerance. When the mass fraction of copolymer was 1.5%, the API filtrate volume (0.7 MPa, 30 min) of fresh water drilling fluid, saline water drilling fluid, saturated-brine drilling fluid and composite-brine drilling fluid were 5.6 mL，9.7 mL，19.0 mL and 16.1 mL. Compared with frequently-used fluid loss additives of SP-8，JT888，MAN104 and HJ-3 in Xijiang oil field, the synthesized copolymer showed the better property in controlling the fluid loss of drilling fluid system. The evaluation of field application demonstrated that the fluid loss additive had good site suitability with potassium calcium based sulphonated polymer drilling and completion fluid system, MMH drilling and completion fluid system, and sulphonated polymer drilling and completion fluid system in Xinjiang oil-field.