There is almost no contribution to system inertia by doubly fed induction generators that are controlled by power electronic converter, which makes system frequency control very difficult. So the study on inertia response function of DFIG is a hot topic in wind power generation at present. A concept of controllable virtual inertia for DFIG is proposed by analyzing basic control schemes of DFIG, and its representation is deduced theoretically. After the inertia control strategies of additional power outer loop and tracing curves switching are described in detail, the control mechanism of virtual inertia is revealed, and its real application is also explained. In the end, the simulation results verify that DFIG will provide the power system with controllable virtual inertia, which is different from the natural inertia of synchronous machine.