The invention relates to clay-free oil-based drilling fluid and a preparation method thereof and mainly solves the problems that emulsion drops and organic soil particles in existing oil-based drilling fluid cause unconventional low pressure and low permeability and large water sensitivity and water-phase activity difference and easily cause solid-phase damage to a stratum, and the oil-based drilling fluid is poor in water invasion tolerance in the prior art. The clay-free oil-based drilling fluid comprises, by weight, 100 parts of base oil, not less than 0.5 part of composite emulsifier which comprises a main emulsifier as shown in formula I and an auxiliary emulsifier, 0.1-5 parts of tackifier, 0.5-5 parts of filtrate reducer, 0-5 parts of alkaline matter. The clay-free oil-based drilling fluid and the preparation method thereof have the advantages that the problems in the prior art are well solved, and the clay-free oil-based drilling fluid is applicable to the drilling completion process of an oil-gas stratigraphic deposit to avoid the problems.