This invention relates to a preparation method of freeze-dried injection of nedaplatin.nedaplatin freeze-dried injection prepared with the inventive method can be used as therapeutic drug of cancer. The invention improves the stability of nedaplatin during lyophilization and shortens the period of lyophilization through adding ethanol during preparation process. the preparation method comprises adding water for injection 80% of nedaplatin into nedaplatin, stirring for dissolving, adding dextran, stirring for dissolving, determing the content of intermediate, adding ethanol 1-10% of the cumulative volume of the solution, adding water for injection to full dose, filtrating with 0.22 ª–m millipore filter under aseptic condition, encapsulating in sirin bottle, plugging, freeze drying, rolling the opening, testing, and packaging. The optimum amount of ethanol is 1-5% of cumulative volume of the solution.