Preparation of acyclovir 2/3 hydrate comprises mixing acyclovir with water at weight ratio of 1:5∼50, dissolving at 50∼100°C, filtering, cooling filtrate at 0∼30°C to precipitate crystal, collecting the crystal by filtration, and drying the crystal at 0∼150°C for 0.5∼24 hours to obtain acyclovir 2/3 hydrate. The preparation process is simple and suitable for industrial production. The prepared product has good and stable crystal form.
