The impact of microgrid accession with high penetration on the large power system security and stability， power quality and dispatching operations is analyzed. The problems are discussed including principle of interaction between large power system and microgrid， optimal planning， operation strategy， relay protection and pattern of electrical market. The key problems arisen from the massive microgrid accession to the large power system are defined. The framework structure and main contents of strategies of large power system coping with microgrid accession are proposed.