In order to meet the higher demand of dynamic reactive power support is put forward in the UHV DC power transmission project, the new-generation synchronous condensers are applied in the UHV DC/AC. However, the excitation system of the new-generation synchronous condensers and the reactive power compensation device of the DC converter station are controlled individually, and there is no reasonable coordination strategy. Therefore, the transient reactive power support capability of the synchronous condenser is analyzed in this paper considering with excitation control characteristics. Then, the transient reactive power demand of the HVDC transmission system is also analyzed. Based on this, the coordinated control strategy of the reactive power compensation device in DC converter station with synchronous condensers are proposed, and a coordinated control system is given in this paper. Finally, the simulation model of the new-generation synchronous condensers access to UHV DC power grids are built based on the PSCAD software. The digital simulation results validate the proposed coordinated control strategy.