Abstract: The eccentric BHA is consisted of an eccentric sub and a certain amount of drill collars, and the placement of the sub various. Its deflection controlling mechanism and stress distribution have been studied and tested in the laboratory condition, and field testing and application carried out in Jurong area of Jiangsu, in drilling the abrupt structures in the Middle Paleozoic Era and the Cenozoic Era. Good results have been obtained, as the hole deflection was effectively controlled and penetration rate was also significantly improved.