Abstract: In the design of casing string,the calculation of external pressure on casing is very important.In the Drilling Manual(Party A),the overburden pressure of the formations is taken as the external pressure on casing in the creep formation section,while the standards of The Structure and Strength of the Casing String Design(SY/T 5724-2008)sets the external pressure of casing as a function of rock Poisson’s ratio.Combining drilling process and rock mechanics theory,the external pressure on casing in the creep formation section is analyzed.The result shows that the formula for effective external pressure on casing in creep formation section in the standards of The Structure and Strength of the Casing String Design(SY/T 5724-2008)is incorrect.It is recommended that the formula for effective external pressure on casing in creep formation section in Drilling Manual(Party A)be used in casing design.