To make the wind power generation support inertial response becomes a necessary trend for the future development of wind power. However, the inertial response of wind power generator is very different from the synchronous generator (SG), which presents high order inertia response characteristics. The reason for the high order inertia response has not been revealed yet. To deal with it, we study the response process of the wind power generator from its power-internal voltage relationship. On this basis, we introduce the concept of the gearbox to investigate the difference between the wind power generator and the SG. It is revealed that the high order characteristics of the gearbox determine the high order inertia response. Then the characteristics, causes and impact factors of the high order inertia response of wind power generation are studied and concluded. Finally the time domain simulation validates the results.