With the access of high-density distributed power generation,the inertia level of power grid continues to decline. Especially under the condition of independent power supply of new energy,the inertia level of power grid is far lower than the minimum inertia demand. By applying the virtual DC motor control strategy to the port converter,the system inertia can be flexibly adjusted by using the control parameters to reduce the inertia difference of the power grid and enhance the stability of the power grid. However,the magnitude of the virtual equivalent inertia of the control method is not clear,and the influence of the virtual inertia on the equivalent inertia needs to be further explored. Therefore, based on the virtual DC motor model,the small signal model is used to analyze the influence of virtual inertia and equivalent inertia on the dynamic characteristics of the system,and the equivalent relationship between them is pointed out. Secondly,based on the corresponding relationship between the virtual inertia and the equivalent inertia,an equivalent inertia calculation method which uses the voltage curve similarity measured by Frecher algorithm is proposed to determine the equivalent inertia of the virtual DC motor control. Then,the correlation constraint and curve feature constraint are introduced,and an improved Frecher algorithm is proposed to ensure the correctness of the equivalent inertia calculation. Finally,the optimal fitting method is used to realize the function of the equivalent inertia relationship,and the quantitative relationship between the virtual inertia and the equivalent inertia is given. The simulation results verify the effectiveness of the proposed equivalent inertia calculation method.