Abstract: Air drilling technology is being concerned more and more，aiming at the phenomenon of serious erosion-wear of drilling tool（BHA erosion） in air drilling，in this article，explaineding the essence of erosion is firstly done，
subsequently，comparative analysis among the influential theory models of erosion-wear carried out，then，three erosion theories for BHA erosion in air drilling educed，with explaining these theories detailedly.These three pieces of theories from different aspects are used to interpret the mechanism of BHA erosion during air drilling.Micro-cutting theory is suitable for explaining cutting condition of low impacting-angle bitcuttings.Deformation wear theory，from the angle of changing energy and deforming process，is used to discusses the mechanism of erosion.The secondary erosion theory interprets the erosion mechanism more overall because it considers the influence of bitcuttings′ breakage to erosion on the surface of drilling tool.At last，a qualitative analysis of main external factors，such as bitcuttings diameter，bitcuttings shape，impacting velocity，impacting angle，downhole temperature，which have great effection on BHA erosion is conducted.

Key words: air drilling, BHA erosion, erosion theory, erosion factor, qualitative analysis