The invention relates to a normal temperature leather diaphragm passivating agent before spraying and coating a steel workpiece, comprising the raw materials by mass percent: 2-8% of phytic acid, 0.1-3% of tartaric acid, 0.3-2% of citric acid, 2-6% of sodium benzoate, 4-8% of triethanolamine and the rest of tap water. A preparation method comprises: adding the metered tap water into a reaction kettle, starting a stirrer under the normal temperature and pressure, and stirring at the speed of 450-550r/min; then, sequentially adding the tartaric acid, the sodium benzoate, the triethanolamine, the citric acid and the phytic acid, continuously stirring for 20-40min after adding every material, and continuously stirring for 0.5-1.5h after adding all the materials; and finally, checking by sampling and packaging until the solution in the reaction kettle is even light purple transparent water-soluble liquid. The passivating agent can be used under the condition of normal temperature, and has the advantages of simple technique, convenient operation, no need for a heating device, low production cost, no pollution, simple technology of the preparation method, easy control and convenient operation.