To address the issue of wellbore instability resulted from hydrate decomposition during offshore drilling for natural gas hydrate， this paper developed a dual-purpose additive which possess both hydrate decomposition inhibition and filtration reduction properties. It was synthesized using 2-acrylamido-2-methylpropane sulfonic acid， dimethyldiallyl ammonium chloride and dimethoxy methylvinylsilane-modified cellulose as the starting materials （named as “CAD”）. The molecular structure was characterized through infrared spectroscopy. Thermal analysis revealed that the product decomposition initiated at around 290°C， demonstrating commendable thermal stability. The hydrate decomposition evaluation experiments indicated that the presence of 1% of the dual-function additive extended the total decomposition time of hydrates by approximately 1-fold and reduced the hydrate decomposition rate by 19.8%， which highlights the outstanding performance of hydrate decomposition inhibition. Filtration in freshwater-based slurry and 5 wt% NaCl saline-based slurry measured to be 6.8 and 8 mL respectively， signifying effective filtration reduction capabilities for hydrate reservoirs. The filtration for the freshwater-based slurry amounted to 6.5 mL at low temperatures， indicating the dual-function additive has favorable performance of low-temperature rheology. All these research offers substantial support for the advancement of high-performance drilling fluids designed for natural gas hydrates.