A novel type of Co–Ti–Mo conversion coating (CoTiMoCC) was prepared on LY12 aluminum alloy, furthermore the optimal formula and the effect of conversion time (CTI) were studied in detail. The micromorphology and phase compositions were systematically investigated by scanning electron microscopy and X‐ray photoelectron spectroscopy, and the corrosion resistance was determined by dropping test, electrochemical tests, and immersion test. Results reveal that the best formula of CoTiMoCC is Co2SO4: 2.0 g/L, H2TiF6: 1.5 ml/L, and Na2MoO4: 1.0 g/L and the corresponding optimal CTI is 16 min. Significantly, the micromorphology of CoTiMoCC under the best conversion condition is relatively smooth, continuous, and dense, and the major elements on the surface are Co, Ti, Mo, Al, O, and F. The phase compositions mainly consist of CoO, Co3O4, MoO3, Mo2O5, TiO2, and Al2O3, as well as a small quantity of AlF3·3H2O and Na3AlF6. Compared with the Al alloy matrix, CoTiMoCC acquires an order of magnitude lower Icorr and thus significantly reduces its corrosion rate, which is also confirmed by electrochemical impedance spectroscopy results. Further study of immersion test exhibits that the corrosion resistance of CoTiMoCC is approximately five times that of Al alloy matrix, and no obvious corrosion spots are generated on its surface.