The virtual synchronous machine is a technical means to improve distributed energy grid connection compatibility. Currently, most research on sliding mode control of a virtual synchronous machine is only for single-machine systems. The single-machine system does not need to consider the coupling effect between the virtual inertia and other parameters in the multi-machine system. As a result, the existing sliding mode variable structure control strategy is not suitable for multi virtual synchronous machine systems, and the system frequency may oscillate when the power fluctuates. In this paper, a tuning criterion of virtual inertia of multi virtual synchronous machines is proposed, and an adaptive sliding mode variable structure control strategy is proposed based on the tuning criterion. This can make each unit in the system output have stable frequency in the disturbed state. The strategy stabilizes the output frequency of multi virtual synchronous units when the power fluctuates, to avoid system frequency instability. Finally, the correctness and effectiveness of the theoretical analysis are verified by building a PSCAD simulation model. The proposed control method can improve the stability and robustness of the multi-virtual synchronous machine system.