The invention relates to the technical field of metal surface treatment, and discloses a trivalent chromium passivator for zinc coating and a preparation method thereof. The trivalent chromium passivator comprises the following raw materials: 60-75g/L of chromium chloride, 10-15g/L of oxydol, 10-12g/L of cobalt nitrate, 5-8g/L of formic acid, 12-25g/L of potassium fluoride, 0.8-1.2g/L of nano silicon dioxide, 0.5-1g/L of surfactant, 0.5-1g/L of corrosion inhibitor and the balance of water. According to the invention, the formula is reasonable, and a prepared passivating film is compact in surface structure, has no obvious crack, is favorable in adhesive force and meets the requirement for product performance; through a 72-hour salt spray test, the corrosion area of a metal spare part passivated by the trivalent chromium passivator is only 3.6%; and the preparation method disclosed by the invention is simple and low in production cost.