Natural polysaccharide polymer modified materials such as cellulose and starch have been widely used in drilling fluids, but the conventional modification methods have limited space to improve their properties such as temperature resistance, salt resistance and rheology. In this paper, Flaxseed Gum (FM) was modified by carboxymethylation using the solvent method, and a treatment agent with multifunctionality, Carboxymethyl Flaxseed Gum (CMFM), was prepared. The successful synthesis of CMFM was confirmed by infrared spectroscopy test and substitution degree determination, and its EC50 value was 4.68×104 mg/L, which was non-toxic and environmentally friendly. The inhibition, rheology, temperature and salt resistance of FM and CMFM were evaluated, and the experimental results showed that the swelling rate of bentonite in FM and CMFM aqueous solution was 28.05% and 27.52%, respectively, and the recovery rate of shale was greater than 80%. 0.5wt% CMFM drilling fluid was aged for 16 h at 120 ℃, and the apparent viscosity, plastic viscosity, and kinetic shear force reached 14.5 mPa·s, 8 mPa·s and 7.2 Pa respectively, and the API filtration loss was 10.7 mL, which has good performance of high temperature resistance. Compared with sulfonated asphalt FT-1, amphoteric polymer filter loss reducer JT888, amphoteric polymer encapsulated filter loss reducer FA367, polyquaternary ammonium salt (large cationic) and vinyl monomer polycopolymer PAC-141, CMFM can increase the viscosity of drilling fluids, maintain a better performance of shear dilution, and maintain a good rock-carrying capacity. The molecules of CMFM have a good resistance to high temperatures, and have good resistance to high temperature, The CMFM molecules form a self-assembled space structure driven by van der Waals force, electrostatic effect, hydrophobic interaction, hydrogen bonding and other forces, adsorb and wrap on the clay particles, changing the original network structure and strength, thus increasing the yield stress of the drilling fluid and enhancing the network structure formed by clay particles and polymer molecules, which is conducive to the suspension of rock cuttings and aggravating materials. In addition, after 20wt% NaCl contamination, the API filtration loss of base slurry added with CMFM was 18.8 mL, which indicated that CMFM possessed stronger filtration loss reduction ability and salt resistance than FM. The drilling fluid system formulated with 4wt% bentonite + 0.5wt% SK-2 + 1% SMP-1 + 1% sulfonated asphalt had lower loss of filtration and better salt and high temperature resistance with the addition of CMFM compared to the drilling fluid with FM. CMFM can be used as a class of multifunctional treating agent with good performance of anti-temperature, inhibition, viscosity enhancement, and filtration loss reduction.