One of the main features of microgrids is their ability to operate in both grid-connected and islanded modes. Switching between microgrid operating modes can lead to significant voltage and frequency fluctuations, which can seriously threaten the stability of the entire system. Seamless switching control strategy is the key to ensuring stable and reliable operation of a microgrid. To solve the problems that traditional seamless switching control strategy is susceptible to interference and poor dynamic stability, a strategy based on an improved linear secondary regulator is proposed, one which includes grid-islanding and islanding-griding smoothing regulators. The grid-islanding smoothing regulator improves the system dynamic performance by providing more damping and compensating for transient voltage drops at the inverter output through an improvement to the traditional voltage control loop. Also, by improving on the traditional sag control strategy, it can adjust its sag coefficient according to changes in the active power of the system and is able to reduce the frequency deviation to the desired level in the presence of disturbances. The islanding-grid smoothing regulator considers the internal control loop and PLL dynamics and improves the traditional current control loop according to the state space model under the grid-connected control strategy. This can ensure the synchronisation of the voltages on both sides of the PCC and the stability of the microgrid frequency. Finally, a small-signal analysis of the proposed control strategy is carried out, and the potential impact of the islanding detection algorithm on the control strategy is investigated, highlighting the robustness of the proposed strategy and verifying that the proposed control strategy can achieve smooth and stable switching between microgrid operation modes.