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With the rise of volume fracturing concept and its application in new wells, it has been shown that increasing fracturing volume is an effective measure to improve production and achieve long-term stable production of ultra-low permeability reservoirs in Ordos Basin. However, refracturing requires running liner, which makes the flow passage smaller and can't reach the volume of new wells. The challenge is how to design the tubing combination and develop matching tools to further improve refracturing volume in 5.5″ casing. The objectives of this study are to design a combined mechanical isolation liner and carry out fine-staged large-volume refracturing with a rate of 8 m3/min in 5.5″ casing. In this study, researchers have designed the liner, optimized refracturing tools and wellhead equipment by comprehensively considering reservoir pressure, wellbore, available tubing combinations and friction of fracturing fluid. Downhole microseismic technology is used to monitor refracturing microseismic events, and high-precision 3D scanning technology is used to detect and analyze the wall wear of liners after refracturing. Field tests have been carried out in Ordos ultra-low permeability horizontal wells and achieved perfect effect. This research provides a reliable solution and technical reserve for volume refracturing of ultra-low permeability reservoir in horizontal wells, and verifies that large-volume plays an important role in improving refracturing effect in the test area.