Abstract: Some horizontal wells in the ultra-low permeability oil reservoirs of the Changqing Oilfield have low first stimulation degree, and the double-packer one-layer volumetric fracturing technology of horizontal well was researched and developed in the early pilot test, but the double-packer one-layer process faces the problems of multiple trips, long blowout time, and channeling outside casing, which seriously restrict its field construction efficiency. In this paper, a new wellbore was reconstructed by means of prefrac energy supplement, gel based circulation loss reduction, Ø114.3 mm casing, resinoid based annulus sealing, the reservoir production increase potential was evaluated, fracture deployment and fracture parameters of old and new fractures were optimized, and the small-diameter soluble bridge plug was developed, so as to form the casing-in-casing wellbore reconstruction and refracturing fracturing technology of horizontal well. It was tested on site in Well CP50-15, where 1500 m Ø114.3 mm casing was successfully run into the hole and the annulus was sealed, ensuring good cementing quality. 26-stage fracturing was completed by using the plug-perforating combined fracturing process, and the construction rate was 3 stages/d. After it is put into production, the controlled blowout production is carried out, and the daily oil production rises from 1.9 t to 15.4 t. This technology provides a new idea for the EOR of ultra-low permeability oil reservoirs.