In order to obtain oil-based drilling fluid with excellent performance at low oil-water ratio suitable for shale gas horizontal wells，the effect of oil-water ratio on the particle size and stability of water-in-oil emulsions was studied by using Turbiscan Lab type dispersion stability analyzer，and the effect of oil-water ratio on the performance of oil-based drilling fluid was researched. The research results showed that particle size of the water-in-oil emulsions increased from 5.65 μm to 8.21 μm when the oil-water ratio decreased from 90∶10 to 60∶40，and the variation range of the reflected light intensity indicated the poor stability with decreasing oil-water ratio，which meant that the stability of the water-in-oil decreased with the decrease of the oil-water ratio. At the same dosage of additives，the plastic viscosity and yield point of the aged oil based drilling fluid increased respectively from 16 mPa·s to 55 mPa·s and from 7 Pa to 30 Pa，the emulsion breaking voltage decreased from 1211 V to 303 V，when the oil-water ratio decreased from 90∶10 to 60∶40. Through increasing the amount of emulsifier and reducing the amount of organic clay，an oil-based drilling fluid with excellent performance at low water-oil ratio was formed，which had been successfully used in well Jiaoye 54-3HF，well Jiaoye 25-2HF，well Jiaoye 54-1HF，and so on.