Abstract: This article emphasizes a prediction of overbreak,the unstable wedge, for openings in jointed rock mass. Firstly, the basic concept of overbreak in opening, its main cause, and the estimating method are described. Secondly, the calculation softwares and their characteristics for opening overbreak are analyzed and compared. Then, a calculation software edited by the authors is introduced. Finely,a calculation case is given, from field investigation to statistical analysis of the parameters, a general formula for an unstable wedge is inferred; after that, the main factors affecting the unstable wedge are shown. The prediction result provides a basis for treating the overbreak.