The inertia of the virtual synchronous generator (VSG) is an important parameter of the micro-grid inverter. In the process of identifying VSG inertial parameters in existing literature, in order to avoid the data divergence during the recursive process, the output data needs to be pre-processed by filter. However, experiments have found that the cut-off frequency of the output filter will affect the final identification result of the recursive algorithm. Aiming at the above problem, this paper firstly analyzes the mechanism of the filtering method affecting the identification results, and then, proposes an identification strategy which performs recursive calculation after equivalent filtering of input and output data. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed strategy are verified by the simulations and experiments.