The invention relates to an ultralow-density oil-base drilling fluid. The invention is characterized in that the ultralow-density oil-base drilling fluid is prepared from the following raw materials: base oil, a density reducer, a suspension stabilizer, an emulsifier, a tackifier, a fluid loss additive, a flow pattern regulator, an alkalinity regulator and a calcium chloride water solution. The base mixed oil is prepared by the following steps: mixing 3# white oil, 5# white oil and natural gas prepared oil, adding a flash point enhancer while stirring, and standing for 0.5-1 hour; and stirring at low speed at the temperature of 30-40 DEG C, and slowly adding oil-soluble nano fumed silica while stirring, wherein the addition process is controlled at 2-3 hours. The ultralow-density oil-base drilling fluid has the advantages of favorable emulsifying stability, favorable rheological properties, high inhibition property for mud shale, high emulsion-breaking voltage, low filter loss, low density (0.75-0.90 g/cm<3>), and high sand carrying and pressure bearing capacities, is beneficial to cleaning the borehole and enhancing the mechanical drilling speed, solves the problem of severe lost circulation in the low-pressure formation and long-section shatter zone formation, and is especially suitable for low-pressure oil gas pools and depletion water-sensitive formations.