The invention provides an oil-based drilling fluid used for depleted reservoir oilfield and adopting crude oil as a continuous phase. The drilling fluid comprises crude oil, a salt solution and a treatment agent, wherein a volume ratio of the crude oil to the salt solution is 70-95:5-30; the treatment agent comprises an emulsifier, a wetting agent, organoclay, a filtrate reducer and a pH adjusting agent, and 1.5-2.5g of the emulsifier, 0.5-0.8g of the wetting agent, 1-3g of organoclay, 3-6g of the filtrate reducer and 1-3g of the pH adjusting agent are added to every 100mL of the crude oil and the salt solution; and the crude oil is crude oil recovered from an adjacent well. The oil-based drilling fluid adopting crude oil as a continuous phase has same borehole wall stability, inhibition and lubrication performances with routine oil-based drilling fluids, has cheaper and more easily available continuous phase than the routine oil-based drilling fluids, and can greatly reduce the drilling cost brought by an expensive continuous phase.