Abstract: Some technical challenges are often encountered in deepwater drilling such as borehole instability through complex formations and the rheological regulation of drilling fluids under conditions of large temperature differences. Thus it is necessary to develop high temperature resistant and strong inhibitive water-based drilling fluids for deepwater drilling. Taking acrylamide, alkyl quaternary ammonium salt and 2-acrylamido-2-methylpropanesulfonic acid as the monomers, a low relative molecular mass polymer encapsulating agent for deepwater drilling was synthesized through aqueous solution polymerization. Then, taking this agent as the main treatment agent, and other treatment agents, a deepwater high temperature resistant and strong inhibitive water-based drilling fluid was prepared. The results of laboratory evaluation demonstrated that the low relative molecular mass polymer encapsulating agent has little effect on the rheology of drilling fluid and in fact exhibits strong encapsulating ability. It presents a good low temperature rheology and high temperature resistance at 160 °C. Its filtration loss at high temperature and high pressure is within 9 mL, the triple-rolling cuttings recovery rate is over 70%, and the encapsulating ability is strong enough to resist fouling of 25.0% NaCl, 0.5% CaCl₂ and 8.0% poor clay, respectively. This drilling fluid has been tested in deepwater drilling in the South China Sea, and achieved good on-site application effects, which solved the technical problems such as low temperature thickening and borehole instability, and also the potential for the promotion of future applications.