Deep formations in the Xihu area of East China Sea are characterized by strong abrasiveness， poor drillability， low ROP and abnormal high temperature and pressure. Through continuous exploration and practice of casing program optimization technology， a relatively fixed exploration casing program has been formed for the East China Sea in recent years. However， further increasing of drilling rate for 444.5mm surface hole has encountered a bottleneck， and there is still room for optimization of casing program for atmospheric deep wells. With the continuous progress on drilling technology and the need for oil and gas exploration and development in the East China Sea， the number of highly deviated wells and horizontal wells is increasing， while it is difficult to run 177.8mm liner in 215.9mm horizontal section of the development well. Therefore， it is necessary to further optimize the casing program in Xihu Sag of East China Sea. To overcome these difficulties， the surface hole size optimization technology， the liner size optimization technology of the development well and the casing program simplification technology of the deep well have been studied and formed. The field application indicated that the drilling speed was improved significantly， which lays a foundation for further optimization of the casing program in subsequent wells. It also reduced the probability of complex conditions in the deep well section， increased the annulus clearance of exploration wells between casing and wellbore， and provided convenience for subsequent operation； meanwhile， it can feed new thought for the well structure design of deep and ultra-deep wells under complex geological conditions.