Abstract: To solve the problems of poor borehole stability and frequent downhole malfunction in original casing program designs in Block Xihu of East China Sea,optimization method took conventional exploration well design principles as reference in which geological setting position and formation pressure distribution 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 intermediate 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 downhole problems,avoided drilling accidents and achieved safe and efficient drilling operations.