Disclosed are a method for preparing crystals of IsPETase protein, a method for screening an IsPETase protein activity regulator and IsPETase variants using a conformation of the protein crystal, a method for screening, IsPETase variants with increased PETase activity, and a method for decomposing PET using the variants. According to exemplary embodiments of the present invention, it is possible to determine a method for effectively preparing a crystal of the IsPETase protein and to obtain the resulting crystal thereof. Further, according to exemplary embodiments of the present invention, it is possible to identify a tertiary structure of the IsPETase from the crystal thereof and to prepare the variant with an increased PETase activity based on this structure. The IsPETase variant may be used effectively in the PET decomposition field.