Abstract: Cementing is a key link in cementing operation, and its success or failure will directly affect the life of oil/gas wells. For small clearance in ultra-deep wells, the design of cementing job is most important, which will directly affect the cementing quality. Due to great well depth and small annular space, the cementing operation involves large friction drag, high pump pressure and poor displacement efficiency during usually induces fluid loss and contamination, which will severely affect the safety of cementing job. In view of the above problems, technical research has been performed and a number of techniques have been presented such as optimizing the design of annular fluid and string structure, the design of anti-contamination prepad fluid, the design of efficient spacer fluid, the design of anti-contamination experiment and the design of cementing and displacement parameters in conjunction with fluid rheology. The application of this technology not only ensures the safety of cementing operation, but also improves the cementing quality in small clearance in ultra-deep wells. The Shuangtan-1 Well cemented with ?127 mm liner was taken as an example, to show the technical measures of safety cementing in small clearance ultra-deep wells and the application effect; field application indicates that this technology is applicable and can ensure safety of cementing operation. The cementing quality is also good.