Abstract: Under low temperature and high pressure, deepwater drilling fluid tends to generate natural gas hydrate with the invasion of natural gas, which deteriorates the performance of drilling fluid and blocks pipelines. And the common method to solve this problem is to add hydrate inhibitor. The dosage of salt alcohol (traditional thermodynamic inhibitor) is over 20%, so it has the disadvantages of high cost and serious pollution. Therefore, it is necessary to research and develop a new kind of kinetic inhibitor. A long-side-chain terpolymer inhibitor YHHI-1 was synthesized by introducing a kind of long-chain monomer into the biopolymer. The polymer was characterized by conducting infrared spectrum and elementary analysis in laboratory, and its inhibition performance was evaluated by using pressure vessel. In addition, the influences of different dosages on the low-temperature aging performance and hydrate formation time of base slurry and experimental slurry were investigated. It is indicated that viscosity-average molecular weight after 6 h synthetic reaction is about 120 thousand. Infrared spectrum verifies that there are expected functional groups in the structure, and elementary analysis reveals that monomer reaction degree is about 100%. The crystallization inhibition time of 2% YHHI-1 solution is up to 120.52 min. Adding 1.0% YHHI-1 into 5% bentonite slurry can reduce the API filtration loss of drilling fluid below 5 mL and improve the low-temperature aging performance of drilling fluid remarkably. When the dosage of polymer YHHI-1 is increased to 1.5%, the API filtration loss of the simulated seawater drilling fluid is reduced to 3 mL, the yield point and plastic viscosity ratio after low-temperature aging is 0.5 Pa/(mPa·s), and no performance changes suddenly. The hydrate formation experiments of 5% bentonite slurry and simulated seawater drilling fluid with different dosages of polymer YHHI-1 indicate that the period without hydration formation is over 5 h if 1.0% YHHI-1 is added into the base slurry; and it is over 3 h if 1.5% YHHI-1 is added into the experimental slurry.