Abstract: Yuanba Area with complex geological conditions faces many drilling difficulties and a number of uncertain factors in which most circulation loss and overflow takes place in Ziliujing formation and upper Shaximiao formation.In view of problems such as shallow casing set depth and interaction in the open hole formation,etc.in previous drilling activities in this area,the casing program of deep wells was optimized with improved casing structure,selection of seal points,deeper casing setpoint and larger casing size for the lower formation.This optimized well structure can not only isolate complex formations effectively,but also create favorable conditions for air drilling and application of other new techniques and efficient development in later time.Field application shows the probability of leakage for each well dropped from 4.4 to 1.9,overflow from 1.6 to 0.4,average air drill footage increased by 51.26%,and average drilling speed increased by 3.64%.The optimized well structure resulted in increased drilling speed and less drilling cycle,which met the requirements for safe and efficient drilling of Yuanba ultra-deep wells.