Abstract: The goal of this study was to eliminate well hazards and drilling problems associated with wide pressure fluctuations encountered in complex strata in the Ying-Qiong Basin of the South China Sea. This basin is characterized by its high temperature and high pressure strata, complex geological structure and narrow drilling mud window. In order to prevent complex downhole events such as circulation lost, overflow and even blowout induced from pressure fluctuations during casing RIH, a calculation model of pressure fluctuation during casing RIH was established on the basis of a full investigation over the common casing program in the Ying-Qiong Basin of the South China Sea, and the optimization over the casing-wellbore gap was conducted by combining annulus pressure analysis during casing RIH and cementing job under different factors. The casing-wellbore gap of the fifth spud section of Well LD10-2-x subject to narrow safe mud window was optimized, and reasonable gap in that section was defined to be 15.00-19.55 mm. The actual casing-wellbore gap in Well LD10-2-x was 17.45 mm within the reasonable gap range, and the casing was run in the borehole with no downhole events such as circulation lost and overflow during the cementing. This indicated that the casing-wellbore gap could be optimized according to surge pressure during casing RIH, as well as annulus pressure during casing RIH and cementing, which would provide a reference in the casing program design in the Ying-Qiong Basin of the South China Sea.