Three phase double mode inverter system was researched in this paper, whose input was battery bank system with PV generation. This system can be operated in stand-alone mode by connecting to local load and also in grid-connected mode by paralleling with grid. When in stand-alone mode, system worked as voltage source, while when in grid-connected mode, the output current of inverter was controlled following with grid. A transition algorithm to restrain current inrush and fast detection of switch and grid status make sure the seamless transition between grid-connected and stand-alone. This inverter system has advantage of simple structure and easy to implement, so that it was appropriate for micro-grid. .