Abstract: Casing program design of mega-extended-reach wells is complex,and in order to design in reason its casing program,based on the principle of pressure balances in the casing program design in the vertical wells,the relationships among pore pressure,fracture pressure and annular pressure loss in the long open-hole section with high inclination in mega-extended-reach wells are discussed.The formulae are deduced for calculating the maximum depth of casing point and the maximum annular pressure loss permitted.The research shows that the most important and manageable factor is annular pressure loss.With the formulae deduced in this paper,the instance of the design of casing program and the management of ECD in a mega-extended-reach well is analysed.The method provides a theoretic gist for mega-extended-reach well.