A database transforming method suitable for transforming a first database of a host into a second database of a mobile electronic device is provided, wherein the first database has a plurality of original fields. First, a plurality of main fields and a composite field are created in the second database. The main fields are respectively corresponding to at least one of the original fields. The main fields and the composite field are used for displaying data in the original fields. Data displayed in the composite field is separated by separators. Next, data in each field of the first database is written into the corresponding main field or the composite field of the second database. In addition, a compressible database structure and a database transforming system are also provided in the present invention.