Abstract: During the drilling process of the 812.8mm ultra large wellbore in the upper part of the ultra-deep well SDC-1 deployed in the northwest of Sichuan Basin, technical difficulties such as difficulty in rock cuttings migration and high requirements for stable drilling of large sections of shale walls were encountered. A drilling fluid system suitable for this well section was studied. By adjusting the flow pattern parameters of drilling fluid, assist in sand carrying; Introducing dual salt inhibitors with organic and inorganic salt coordination, embedding them into the gaps between clay crystal layers, reducing drilling fluid activity, and synergistically inhibiting shale with coating agents; A polymer drilling fluid system with high inhibition and strong sealing has been formed by optimizing multi particle size sealing agents. The indoor evaluation results show that the drilling fluid system has good rheological properties, with a dynamic plastic ratio of over 0.60. The primary shale recovery rate is 99.54%, the secondary recovery rate is 97.23%, and the interlayer spacing of bentonite is 1.30 nm; The API filtration loss is controlled within 3.0 mL, which has a good inhibitory and blocking effect. On site applications have shown that the polymer drilling fluid system with high inhibition and strong sealing can effectively suppress the collapse of shale in the Jianmenguan and Penglaizhen formations, without any underground accidents, providing technical support for the smooth construction of SDC-1 well. The research results provide technical reference for the smooth construction of other ultra large sized boreholes.