Abstract: For microgrid（MG） system with master-slave control strategy, seamless transfer between grid-connected and islanding operation remains a technical barrier, which needs to be solved urgently. In this paper, the structure and operation principle of the MG was firstly investigated. According to the dynamic behavior exhibited in the mode transfer process, an improved proportional-integral（PI） controller for voltage-loop was proposed. By using energy conservation theorem, the output value of the new PI controller can be calculated and preset for next operational mode before mode transfer. Thus, the vibration and overshot problem of local AC bus could be well reduced, even eliminated. Also, a novel phase-locked loop（PLL） for MG application was presented. The phase angle of AC bus can be maintained smoothly and successively during different kinds of mode transfer condition, which makes the seamless transfer between grid-tied and islanding operation possible. Then, optimized parameters of the controller were derived for main converter with mathematical model and engineering experience. Both simulation and experimental platform were established, simulation and experimental results verify the effectiveness and superiority of the proposed strategies.