The dynamic motion of system center of inertia (COI) can effectively indicate the general trend of a power system dynamics. Based on this, the developed COI dynamic model considering integrated doubly-fed induction generators (DFIGs) is proposed. With the impacts of DFIGs on the key factors in COI dynamic motions deduced, the influence mechanism of DFIGs on the power angle accelerating speed (PAAS) of system COI is then deeply investigated. If DFIGs are integrated without displacing synchronous generators, the change in PAAS of system COI is mainly determined by the capacity and transient power response of integrated DFIGs. And when synchronous generators are displaced, the change in PAAS of system COI is affected jointly by the DFIG transient power response and the proportions of the inertia and capacity of displaced synchronous generators in the origin system. The theoretical analysis is effectively verified and illustrated by simulation studies.