The invention discloses a chrome-free passivant for a hot-dip galvanized steel sheet and a preparation method thereof. The passivant comprises acrylic resin, phytic acid, sodium silicate, ATMP, thiourea, titanium trichloride hydrochloric acid solution, tannin, succinic acid, 1-octanesulfonic acid sodium salt, benzothiazole, silica sol, triethoxyphenylsilane, HEDP, vanadyl acetylacetonate, polyethyleneimine and deionized water. The preparation method of the chrome-free passivant comprises the steps of firstly adding the acrylic resin, the phytic acid, the sodium silicate, the ATMP and the thiourea into the deionized water, heating and stirring, cooling down the mixture, then adding the titanium trichloride hydrochloric acid solution, the tannin, the succinic acid, the 1-octanesulfonic acid sodium salt and the benzothiazole, preserving the heat and stirring, cooling down the mixture and then adding the silica sol, the triethoxyphenylsilane and the HEDP, preserving the heat and stirring, cooling down the mixture, then adding the vanadyl acetylacetonate and the polyethyleneimine, stirring and obtaining the passivant. The passivant disclosed by the invention does not contain chrome and is environmentally friendly, and the galvanized steel sheet passivated by the passivant has higher corrosion resistance and oxidation resistance.