In recent years, indoor position has been an important role in many applications, such as production management, store management and shelves in supermarket or library. Much time and energy are exhausted because one object cannot be quickly and accurately located. Traditional indoor position systems have some problems, such as complicated software and hardware system, inaccurate position and high time complexity. In this paper, the authors propose an RFID-based collaborative information system, Tagrom, for indoor localization using COTS RFID readers and tags. Unlike former methods, Tagrom works with reference tags and phase of Passive UHF-RFID tags, which improves traditional distribution of reference tags and utilize RF phase replace of traditional RSSI or multipath profile to determine the position of target RFID tags.