In the traditional power system, the inertia time constant of a synchronous generator is an invariant physical parameter. However, with the application and popularization of the virtual synchronous generators, the inertia time constant of the system becomes variable and adjustable, which may affect the small disturbance stability of the interconnected system. VSC-HVDC is taken as the research object and the virtual synchronous generator control is built on it. Then, based on the eigenvalue analysis method, the analytic expression of the low-frequency oscillation attenuation factor for the equivalent networking system is solved, and the degree of influence of the inertial time constant on the stability level of small disturbance is analyzed. In the proposed method, the inertia time constant has a great influence on the small disturbance stability when being in the same order of magnitude with damping coefficient. Finally, the influence of the inertia time constant on the regional low frequency oscillation mode is verified in the classical model of 2-area 4-generator, which has certain reference value for the analysis and control of the regional small disturbance stability of the interconnected power system and the virtual synchronous generator parameter setting.