In the scenario with dynamic varying loads, for the traditional VSG secondary frequency modulation strategy, the integrator continuously accumulates due to the continuous change of frequency deviation, which may lead to poor frequency adjustment effect and even frequency oscillation. For improvement, a secondary frequency modulation method of isolated microgrid, which based on the identification of load disturbance modes and adaptive adjustment of VSG, is proposed in this paper. Firstly, the issues in the traditional VSG secondary frequency regulation strategy of dynamic varying loads is analyzed. Then, the power characteristic data is used to discern the modes of load disturbances in real time. When the secondary frequency modulation is requisite for the microgrid, and the load disturbance is classified the mode of dynamic change, the parameters and virtual governor structure of VSG are changed by an adaptive adjustment. The adjustment is predicated on the deviations of frequency and bounded by the thresholds of secondary frequency regulation and active power change, thus enabling the frequency tracking in the microgrid. Further, a small signal model is constructed to evaluate the implications on system stability prompted by the incorporation of an integrator and the flexible adjustment of VSG parameters. Finally, the feasibility of the proposed frequency modulation strategy is verified by the results of simulation testing. When compared with the other existing strategies under the same conditions, the proposed strategy limits the range of secondary frequency fluctuation of microgrid within 0.0173 Hz, which exhibits significant merits in decreasing the magnitude of frequency offsets.