Abstract: Sulige Gas Field presents low-pressure and low-permeability features. Production of some wells with traditional technology is unprofitable. Implementing drilling fluid technology for sidetracking horizontal wells with Ø139.7 mm casing windows is beneficial for the recovery of abandoned wells. Compared to conventional wellbores, the sidetracking horizontal wells with Ø139.7 mm casing windows face technical challenges such as lubrication, anti-sticking, mudstone collapse in the horizontal section and leaking in narrow annular spaces. Therefore, a CQSP-4 drilling fluid system, with strong inhibition and powerful sealing capacity, was developed. And, the rheological, anti-collapse, inhibitive, and lubricating properties of the drilling fluid system were evaluated indoor. The experiments showed that the cuttings rolling recovery rate reached 95.63% after 12 h at 100 ℃, and after 24 hours of hot rolling at 100 ℃, API filtration loss was 2.8 mL, and the high-temperature and high-pressure filtration loss was 13.2 mL, with a high resistance to cuttings contamination up to 14%. Furthermore, drilling fluid processes such as casing window sidetracking, open-hole section drilling, solid phase control, leak prevention, friction reduction and hole cleaning were optimized. As of December 2022, a total of 18 wells had been field-tested. The drilling cycle reduced by 69.58% compared to 2017-2018, the rate of penetration increased by 102.82%, and the average drilling cycle decreased from 80.5 days to 24.49 days, achieving the goal of speeding up and improving efficiency. This technology has a guiding significance for the construction of similar wells in other regions at home.