Abstract: The design idea of the curved profile of the API long round thread casing connection is put forwarded. And thread contact stress distribution of the tapered thread is compared with that of the curved thread by means of the finite element analysis(FEA)method. Relevant calculation result shows that the curved thread can get the stress distribution optimized,and that such type of thread is easy to be processed,and meanwhile,no more cost is caused,and moreover the gulling rate will be reduced,which means a longer service span.