To solve the problems of poor borehole stability and frequent downhole malfunction in origi‐nal casing program designs in Block Xihu of East China Sea ,optimization method took conventional explo‐ration well design principles as reference in which geological setting position and formation pressure distri‐bution were utilized to determine casing size ,level and setting depth .Based on geological condition ,well control ability and completion operations ,the size and depths of surface casing and completion casing were determined .The optimized casing program is as :φ508.0 mm surface casing around the depth of 600.00 m to cement unstable formations and to set up wellhead with adequate well control ability ;φ339.7 mm interme‐diate casing at the depth of 2 400.00 m to plug complex formations as unconformity and seam ;the φ244.5 mm intermediate casing around the depth of 4 000.00 m to plug the pressure transitional belt ensuring enough pressure capacity when drilling through the reservoir .At the same time ,a φ298.4 mm intermediate casing from well depth 2 400.00 to 4 000.00 m well section for particularly complex formations to avoid low ROP and well testing difficulty in deep slim hole .The optimized casing program have been used in two vertical wells Block Xihu of East China Sea .Compared with adjacent wells with original casing design ,it successfully plugged complex formations ,reduced dow nhole problems ,avoided drilling accidents and a‐chieved safe and efficient drilling operations .%为解决东海西湖区块原有井身结构存在的问题及井壁稳定差、井下故障多等问题，参考海上常规探井自上而下的井身结构设计方法，根据地质必封点和地层压力分布确定技术套管的尺寸、层次和下入深度；根据地质目标、井控能力和完井测试要求确定表层套管和完井套管的尺寸和下入深度。优化后的井身结构为：φ508.0 mm表层套管下至井深600.00 m左右，封固不稳定地层并建立井口，提供充足的井控能力；φ339.7 m m技术套管加深下至井深2400.00 m左右，封固不整合面和煤层等不稳定地层；φ244.5 m m技术套管加深下至井深4000.00 m左右，封固压力过渡带，保证打开储层时具有良好的承压能力；同时，在2400.00～4000.00 m 井段备用φ298.4 mm技术套管封隔特别复杂地层，避免深部出现小井眼井段和储层测试困难的现象。东海西湖区块的2口直井采用了优化后的井身结构，与采用原井身结构的邻井相比，优化后的井身结构成功封隔了复杂地层，有效减少了井下故障，避免了钻井工程事故的发生，实现了安全高效钻井。
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