Abstract: Deflection re?fracturing is an important way of liberating remaining oil in low permeability reservoir in old fields. However, conventional deflection re?fracturing technology has many problems: whether the fracture is deflected or not? How does the deflected fracture propagate? How to get the fracture deflected accurately? Finally, they genenrally cause the unsatisfactory deflecting angle, ineffective treatment, inaccurate effect evaluation, etc. In view of this and on the basis of rock mechanics, fracture mechanics and hydraulic fracturing mechanics, this paper puts forward the concept of“secondary deflection fracturing in fractures”, analyzes and contrasts four kinds of fracture shapes that probably appear in deflection fracturing in fractures. And it is concluded that the best deflecting position is just at the inflection point of horizontal stress difference, which can get maximum communication with reservoirs. Also, applicable reservoir conditions, key techniques and the preliminary design methods are presented. These techniques are of great significance for developing deflection fracturing systems and for converting empirical fracturing treatment to fine fracturing treatment