Abstract: There are two different operation modes for microgrid, which are grid-connected and islanding operation. In order to solve the problem of seamless transfer between these two operation modes, a presynchronization software algorithm for microgrid is firstly designed to make the phase angle of AC bus maintain smoothly, which lay foundation in seamless transfering between grid-connected and islanding operation. Then the basic structure and operation mode of the microgrid are analyzed. According to the dynamic output behavior of voltage loop exhibited in the mode transfer process, an improved controller for voltage-loop in the DC-DC converter and inverter is proposed. The improved controller can achieve reset function of transient output demand between grid-connected and islanding operations to compensate the overshoot caused by power flow mutation in the system for traditional PI controller. Thus, the vibration of DC bus and AC bus can be well reduced during transfer process. In the end, simulation of microgrid with photovoltaic/battery system is built by using matlab/simulink, the result verifies the effectiveness of proposed strategies.