Abstract: In order to achieve mechanical plugging without inner diameter loss while drilling deep and ultra-deep wells in complex formations, a MonoHole expandable casing system with a size of ϕ219.1 mm was developed by selecting tubes with a large expansion rate, and by designing the expandable screws with a large deformation and adjustable expansion tools. The mechanical and expansion properties of the system were evaluated through laboratory tests. The results indicated that the system was characterized by a collapse resistance of 15.1 MPa, a burst strength of 29.3 MPa, and a connection strength of 1 850 kN, which met the requirement of using the system as emergency casing for temporary plugging. The expansion pressure was 18-20 MPa. The expansion was smooth, and the system performs well after expansion. After the downhole functionality and open-hole running of the system were tested, a downhole whole-process expansion test was conducted. The system expanded smoothly, and its inner diameter after the expansion was ϕ245.0 mm, which met the running requirement of tools with a size of ϕ241.3 mm. The results achieved the research objectives to bring about the same-diameter expansion and drilling without inner diameter loss. The research and test results revealed that the MonoHole expandable casing system with a size of ϕ219.1mm could accomplish the same-diameter expansion and meet the requirements of MonoHole expansion and plugging operation. The research results lay a foundation for the field application of the MonoHole expandable casing system.