Smooth switchover of microgrid from grid-connected operation mode to islanded operation mode can guarantee the continuous power supply to important loads. A microgrid model for inverter source is established with MATLAB/Simulink software， which realizes both grid-connected and islanded operation modes of microgrid and their control methods. Then the impacts of control parameters on the spectrum characteristics are analyzed based on the transfer function of controller system. And through time-domain simulation， impact of control parameters on transient process of microgrid operation mode switchover are investigated. According to the analysis above， a microgrid smooth switchover control method based on controller status follow is presented. In the meantime， as viewed from the micro-source capacity matching with load， the three-region smooth switchover control strategies are proposed to effectively reduce the transient oscillations in switchover process of microgrid two operation mode. Finally， the simulation results show the effectiveness of the proposed smooth switchover method and strategies.