In this study, based on the area balancing technique and geometrical relationship, we developed a 2-D model and related equation to calculate pore space of fractures associated with fault development through constructing a detachment void space model in a fault-bend fold and a fracture pore space to void space exchange model. Geological meanings of the main parameters in the equation are discussed. The result indicates that the derivative of the angle between the fault and the bedding surface is the major control factor on the development of fracture pore space associated with fault. The equation created for calculating fault fracture pore space provides a new thought and method for quantitative description of the development of fault fractures.