Abstract: During the drilling of the ultra-large wellbore of ϕ812.8 mm in the upper part of the ultra-deep well SDCK1 deployed in the northwest of Sichuan Basin, technical difficulties such as difficulty in rock cuttings migration and high requirements for wellbore stability were encountered. Therefore, the flow pattern parameters of drilling fluids were adjusted to assist in sand carrying; dual salt inhibitors with organic and inorganic salt were introduced and embedded into the clay lattice space, so as to reduce drilling fluid activity and synergistically inhibit shale with coating agents. A polymer drilling fluid system with high inhibition and strong plugging was formed by optimizing multi-particle size plugging agents. The laboratory evaluation results show that the drilling fluid system has good rheology, with a ratio of yield stress and plastic viscosity over 0.60. The primary shale recovery rate is 99.54%, the secondary recovery rate is 97.23%, and the API filtration loss is controlled within 3.0 mL, indicating a good inhibitory and plugging effect. Field applications have shown that the polymer drilling fluid system with high inhibition and strong plugging can effectively inhibit the collapse of shale in the Jianmenguan and Penglaizhen formations, without any downhole accidents or complex situations, providing technical support for the successful drilling of Well SDCK1. The research results provide a technical reference for the drilling of other ultra-large wellbores.