Abstract: Air drilling，as a member of under balance pressure drilling family，is different from conventional drilling methods.In order to predict the flow characteristics，it is necessary to derive the unified applicable theoretical formula，which can provide basis for choosing compressor and determining drilling methods.The air volumetric flow is the key to guarantee the construction of under balance drilling.Owning to air drilling taking the compressed air as circulated fluid，the circulation system volume flow is closely related to pressure variation，so the pressure calculation becomes the major research emphasis.According to the order of air injection，the pressure formula derivation of drill string and annulus were respectively studied，and took the air drilling as an example to solve the generic solution.According to the boundary condition to ascertain the constant，the law of pressure variation changing with the temperature and depth could be gained.To further compute the air volume flow could provide the solid basis for compressor.

Key words: Air drilling, Normal circulation, Pressure model, Drilling pressure