Abstract: After the initial hydraulic fracturing of the ultra-low permeability oil reservoir, there is a rapid decline in oil production due to the depletion of formation energy and fracture failure. This decline fails to meet the production demands, and the refracturing is needed. In the design of refracturing, the key lies in determining the optimal positioning and number of new fractures. In this study, a numerical simulation model for fracture extension during refracturing of horizontal wells was established, based on the geological characteristics of the Changqing yuan 284 well block oil reservoir and the production data from the initial hydraulic fracturing. A comparative analysis of the reservoir stimulation volume and ultimate developing effect was conducted with varying proportions of new and old fractures. The study found that energy replenishment by water injection before refracturing as a mechanism could effectively improve the reservoir stimulation effect of refracturing. Furthermore, an economic benefit analysis was performed to determine an reasonable proportion of new and old fractures. Simulation results indicated that a favorable condition for the formation of a complex fracture network and increased oil recovery was observed when there were two new fractures by refracturing, with a spacing of 20 m between fractures. As the number of new fractures between two old fractures increased, the formation of a complex fracture network and reservoir stimulation effect were further enhanced. However, a large number of new fractures between old fractures led to mutual interference, resulting in a decrease in the range of production increase during the early stage of development. Nonetheless, as development and production continued, there will be a more thorough reservoir stimulation effect, which is beneficial for stable production in the later period. These research findings offer a theoretical foundation for designing refrcuturing schemes for horizontal wells in ultra-low permeability oil reservoirs.