Abstract: In drilling practice, Because of the lack of understanding of deflecting characteristic of anisotropy formation while underbalanced drilling, a appropriate measure can not be taken accordingly. So research on deflecting characteristic of anisotropy formation while underbalance drilling will play an important role in underbalace drill technology and should be carried out immediately. In this thesis, the stress distribution of bottomhole rock in various pressure difference and formations with various angle of bedding is analyzed by means of finite element method, the influence of pressure difference and formation angle on the stress distribution of bottom hole is founded, the discrimination criteria of inclination trend is established, the effect of underbalance drilling condition on the deflecting characteristic of anisotropy formation is further discussed. The result shows that the negative pressure differences while underbalanced drilling amplify the asymmetry of stress distribution and strengthen the deflecting tendency of formation. This will be more remarkable with the increase of angle of bedding.