Abstract: Wellbore stability in horizontal wells for shale oil reservoirs is very important. The mineral fabric and physical and chemical properties of mudstone in reservoirs were studied by means of electron microscopy scanning, X diffraction, expansion dispersibility testing and rock mechanics testing. The results showed that microcracks were developed in the samples, with clay content as high as 62.24%, mainly of illite and without montmorillonite, and it has the characteristics of weak expansion, weak dispersion, strong oil-wet and weak water-wet. After soaking in water-based drilling fluid for 36 hours, the compressive strength decreased by24.8%. Based on the above research, the performance and supporting measures of water-based silicate drilling fluid were optimized to form the wellbore stability technique in horizontal wells for shale oil reservoirs. The horizontal section of Dongping-1 well is 625 m long, and the first drilling was completed successfully.