Abstract: Hybrid AC/DC power grid is the new form of power grid development. Its feature is vulnerability,controllability, cascading fault, and so on. In China, it is a significant and urgent national demand to guarantee the high quality, high efficiency, security and reliable operation of existent large AC/DC hybrid power grids. To solve the facing problems of large AC/DC power grids safe operation, this article analyzed hybrid grid’s structural characteristics and condensed two scientific problems which had to be solved urgently and a key technology. Existing specific problems were analyzed from three aspects, which were hybrid grid’s steady operation characteristic analysis and control technology, faultcharacteristic analysis and protection technology, and simulation technology, respectively. These problem’s preliminary analysis and potential solution were given. Research direction of large AC/DC hybrid power grids safe operation was indicated. Complexity of these problems and urgency of solving these problems were shown directly by the last simulation case.