The invention relates to a film treating agent for treatment before electro-coating of die-cast aluminum alloy workpiece, containing the following raw materials in percentage by mass: 2-10% of fluorozirconate, 1-13% of nitric acid, 1-5% of citric acid, 1-8% of phytic acid, 0.5-3% of hydrofluoric acid, 0.3-1.2% of ammonium bromate, 3-8% of ammonium bicarbonate, and the balance of tap water. The preparation method comprises the steps as follows: adding metered tap water in a reaction kettle; turning on a stirrer at normal temperature and normal pressure with rotational speed 450-550 revolutions/min; and, adding ammonium bicarbonate, ammonium bromate, fluorozirconate, nitric acid, citric acid, phytic acid and hydrofluoric acid in sequence, completely dissolving and packing. The film treating agent for treatment before electro-coating of die-cast aluminum alloy workpiece produces no toxic or harmful substance during the treating process and has no influence on the operator and the environment; besides, the treatment can be carried out at normal temperature without heating, so that the device is simplified and the energy is saved.