The invention discloses a diesel engine oil and a preparation method thereof. The diesel engine oil comprises, by mass, 1.6 to 2% of a pour-point depressant, 2 to 3% of a viscosity index improver, 5 to 7% of a composite anti-oxidant including a hindered phenol anti-oxidant and a sulfurized alkylphenol anti-oxidant, 11 to 13% of an alkaline clearing agent, 2 to 3% of an oxidation and corrosion inhibitor, 1.5 to 3% of an antiwear agent and 3 to 5% of a dispersing agent, with the balance being base oil, summing to 100%. The diesel engine oil provided by the invention has an appropriate and high base number, excellent sour corrosion resistance, good resistance to abrasion and corrosion, good high-temperature cleaning performance and good antioxidation performance.