Abstract: As to the problem of transient oscillation of microgrid during process switching from grid-connected operation to islanded operation, a novel control method of smooth switchover for microgrid based on master-slave control is proposed in this paper. The outer-loop controller and current controller of battery energy storage system are designed as a master unit firstly. Secondly, control strategy of smooth switchover for master unit is put forward based on factors influencing transient oscillation during switchover process. In the end, control strategy of region partition is applied to reduce oscillation caused by the power difference of master unit between slave unit(s) and load margin. Simulation results show that proposed control method of smooth switchover can not only transition from grid-connected operation to islanded operation smoothly, but also avoid transient impact due to the capacity deficiency of master unit.