The invention discloses a chromium-free passivating agent suitable for an aluminum alloy. The chromium-free passivating agent per L is prepared from the following components by mass percent: 50% of amino silane or/and silicane epoxide, 10% of titanium dioxide, 10% of zirconium compound or/and titanium compound, 3% of cupric ions, 5% of starch, 5% of sodium phenylacetate, 3% of polyamide polyethylene glycol, 1% of sodium bicarbonate, 3% of alum, 2% of sulfonated castor oil, 2% of sebacic acid and the balance water. The passivating agent can enhance the thickness and density of a passivation film through different degrees of chelation of various organic phosphoric acids, various metal ions and various nano particles. Meanwhile, the passivation film (an inorganic film) being formed after chelation can show a certain degree of color on the surface of aluminum alloy.