In view of the frequency offset error and large oscillation of microgrids in secondary frequency regulation under islanding mode, an integral coefficient adaptive secondary frequency regulation control strategy with integral coefficient has thus been proposed. Firstly, the integration coefficient is divided into two stages of control through the angular frequency change rate, with the size of the integration coefficient adaptively adjusted according to the requirements of the two stage changes, so as to reduce frequency offset and oscillation during the secondary frequency modulation process, thus improving the stability and reliability of the system. Secondly, in view of the low speed and accuracy in the switching process from islanding to grid connection, a new phase-locked-loop-free pre- synchronization control strategy is to be proposed based on the control strategy in islanding mode. A compensation can be achieved by replacing the distance between the end of the voltage vector output by the inverter and the end of the grid voltage vector instead of the phase difference, with a new grid connection judgment condition proposed, thus eliminating the phase-locked loop (PLL) and root mean square value (RMS) links with the requirement of only one PI regulator, reducing control parameters, improving the speed and accuracy of pre synchronization, and achieving a smooth switching between off grid and grid connection. Finally, the accuracy of the proposed method can be verified by Matlab/Simulink simulation and semi-physical simulation.