Grid-forming converters controlled by virtual synchronous generator （VSG） provide a certain amount of inertia and damping support for new energy sources to be connected to the grid. However，the power coupling generated by the VSG control link causes system frequency oscillation，which can seriously threaten the system safety. To solve the unclear mechanism of power coupling interaction on frequency oscillation when multi-VSGs are connected to the grid，a method of impedance modeling of multi-VSG grid-connected system considering power coupling is proposed. The frequency oscillation characteristics are analyzed by combining with parameters changes. Firstly，the power coupling mechanism of single VSG is analyzed. Then the impedance model of multi-VSG grid-connected system is established which reveals that output angular frequency under multi-VSGs is mainly affected by three parts：its own power，interacting power and grid angular frequency. In addition，a method based on the dynamic relative gain array is proposed to quantitatively analyze the laws of system parameter variations on the interaction effects among VSGs. The study reveals the impact trends of parameter variations in multi-VSG systems on system coupling interactions across high-frequency and low-frequency bands. Finally，the accuracy of the theoretical analysis is verified by time-domain simulation of a three-VSG grid-connected system model constructed in PSCAD.