This paper analyzes the inertia response characteristics of the Permanent Magnet Synchronous Generator(PMSG) based wind energy conversion system (WECS) with Virtual Synchronous Generator(VSG) control. It is found that the inertia response characteristics of the WECS would be influenced by the dynamic interaction of the VSG control loop and the Maximum Power Point Tracking(MPPT) control loop which is widely applied in the wind turbine to achieve maximum power conversion under different wind speeds. The interaction would affects the releasing and accumulating of rotor kinetic energy then may deteriorate the inertia response of the WECS. Different MPPT control structures can result in different dynamic interactions. By analyzing the equivalent inertia time constant, this paper investigates the influence of the MPPT control structures on the inertia response characteristics of the WECS. Simulation results verify the validity of the theoretical analysis.