The invention relates to a melphalan freeze-dried powder injection and a preparing method thereof. The prepared melphalan freeze-dried powder injection is used for treating multiple myeloma, oophoroma, polycythemia vera, local malignant melanoma and soft tissue sarcoma. The melphalan freeze-dried powder injection contains melphalan, utilizes the mixed solvent composed of the tert-butyl alcohol and the injection water in the preparation process, wherein the concentration of the melphalan in the mixed solvents is 10-25 mg/ml; and the volume ratio of the solvents is: 5-50% of tert-butyl alcohol and the balance of injection water. The preparation process comprises the following steps: measuring tert-butyl alcohol, adding injection water, mixing evenly, cooling to 2-15 DEG C, heat preserving, adding melphalan, stirring and dripping 0.5 mol hydrochloric acid to dissolve, filtering, filling, plugging, disking, freeze-drying, pressing plug, out box, tying and packing after quality test qualification.