Abstract: Proneness to well deviation and difficulty in casing running are two bottlenecks to be solved urgently in air drilling through the thick conglomerate formation. Field test data revealed that well deviation control was still challenging even air hammer drilling with a marked control effect on deviation was adopted, and wellbore regularity was poor. This phenomenon is difficult to explain with present theories of deviation control. Existing bottom hole assembly (BHA) force models do not consider the irregularity of the wellbore, and assume that the wellbore is smooth and regular. In this paper, the mechanical model of interaction between the irregular wellbore and BHA was built with finite element method on the basis of field data, and the influence of wellbore irregularity on the mechanical characteristics of BHA was analyzed. The model results demonstrate that the irregular wellbore is easy to form additional fulcrum and shorten the swing distance of pendulum BHA, thus greatly reducing deviation-reducing force. It may even convert the lateral force on the drill bit into a deviation-increasing force, resulting in well deviation control failure. It is confirmed by case analysis that the wellbore drilled by the pre-bent pendulum BHA is regular in air drilling through the thick conglomerate formation, with effective well deviation control and smooth running of casing. Field data indirectly supports the significant impact of wellbore irregularity on well deviation, which should be considered in the BHA mechanical analysis.