The present invention relates to a method for preparing an imipenem active pharmaceutical ingredient, the method comprising the following steps: 1) dissolving crude imipenem in water at 5-25°C to obtain an aqueous solution of imipenem; 2) concentrating the aqueous solution of imipenem obtained in step 1) by passing through nanofiltration so as to obtain a concentrated aqueous solution of imipenem; 3) decolorizing the concentrated aqueous solution of imipenem obtained in step 2) by means of activated carbon, filtering out the activated carbon, and then crystallizing; and 4) separating, washing, and drying the crystal precipitated in step 3) so as to obtain an imipenem active pharmaceutical ingredient. Said method has a simple, compact and controllable process, and at the same time has high product yield, high purity and low costs, and has huge economic advantages.