Abstract: Borehole curvature is one of significant index to evaluate angle build-up capacity and wellbore quality,is very important to realize safety,speedily and efficiently drilling.The wellbore curvature of any point on hole trajectory can be calculated exactly,but more concern were focus on average curvature for drilling engineer.Introduced four calculate method of bending angle,tow of which are completely equivalent and the other two are similar.Recommend average curvature calculate method,from which can get exact average curvature,it has well scientific and rationality.