In order to satisfy the needs of plug and play feature of distributed resources（DRs）， class extension of microgrids is proposed to build the microgrid common information model（CIM） according to CIM extension guideline and microgrid's characteristics. For the established microgrid CIM， the changes to the existing microgrid CIM due to the addition of new DRs to a microgrid with fixed network structure is investigated. Referring to previous studies on application of power system CIM， the method of automatic implementation is explored to modify microgrid CIM after adding new DRs with plug and play feature. The research results can meet the needs of plug and play feature of DRs in a microgrid with fixed network structure.