The invention relates to a method for preparing an imipenem monohydrate crystal (I), which comprises the following steps of: 1) dissolving the crude product of imipenem in water in the nitrogen atmosphere to obtain solution; 2) performing activated carbon treatment on the solution; 3) adding an organic solvent into the solution and reducing the temperature to make the solution in a solid-liquid mixed state so as to form a crystal nucleus; and 4) raising the temperature to melt the crystal nucleus and adding the organic solvent to crystallize and precipitate imipenem monohydrate so as to obtain the imipenem monohydrate crystal. The method for preparing the imipenem monohydrate crystal does not comprise the step of thickening and dewatering or adding acid or alkali to adjust the pH value, so the method is simpler and effective and is easy to control; and the imipenem monohydrate crystal prepared by the method has the advantages of uniform crystallinity, high purity and quick solubility.