The invention relates to the technical field of metal passivators, and particularly relates to a chromium-free composite passivator and a preparation method thereof. The chromium-free composite passivator provided by the invention is prepared from the following components in percentage by mass: 3.0-10% of gamma-methylacryloyloxytrimethoxy silane, 0.1-3.0% of sodium molybdate, 0.1-5.0% of cerium chloride, 0.5-4.0% of phosphoric acid, 0.5-4.0% of acetic acid, 15-20% of water-soluble epoxy resin, 5-15% of water-soluble acrylic resin, 10-30% of water and 20-40% of ethanol. The chromium-free composite passivator is suitable for treating steel, zinc and zinc alloy, or aluminum and aluminum alloy. The chromium-free composite passivator contains the water-soluble high-molecular resins and inorganic corrosion inhibitor, and is mixed with ethanol to form a colorless, innoxious, environment-friendly, transparent and clear solution. At room temperature, the chromium-free composite passivator is applied on the surface of steel, hot-galvanized zinc and zinc alloy, or aluminum and aluminum alloy to generate chemical crosslinking reaction, and is cured to form a film.