Abstract: Vertical volume fracturing, as a new and efficient exploitation mode, is of important significance to rapid yield increase for the reservoirs under repeated transformation. On the basis of rhombus inversed 9-point pattern of ultra-low-permeability oil reservoir, a well group model for research is established herein with time variation of fractures in initial fracturing and the characteristics of complex fracture network under repeated fracturing taken into consideration. In addition, numerical simulation method is adopted for optimization of potential wells under repeated fracturing according to residual oil distribution results, and optimization design for fracture network parameters is conducted by orthogonal experimental method. Results show that, major fracture conductivity affects yield-increasing effects significantly, the length of major fracture and secondary fracture conductivity are in the second place for yield-increasing effects, and the width of fracture network and secondary fracture spacing show relatively low affecting degree. In the scope limited by actual well pattern, well spacing and fracturing process, the best exploitation effect is realized with major and secondary fracture conductivity of 25 μm2?cm and 3 μm2?cm respectively, major fracture length of 290 m, and fracture network width and spacing of 100 m and 30 m respectively. Research results not only provide theoretical basis for repeated transformation of Chang 6 ultra-low-permeability oil reservoir, but also provides important reference for optimization design for fracture network under repeated fracturing of similar reservoirs.