During sending-end faults in the hybrid cascaded HVDC (HC-HVDC) system, the transient voltage drop characteristics under the interaction of the AC/DC hybrid system remain unclear, and the reactive power support provided by the HC-HVDC to the sending-end AC system requires further investigation. To address this problem, the reactive power interaction coupling mechanism between the sending-end AC system and the HC-HVDC is revealed, and the transient voltage mathematical model considering fault severity and duration is established. Under the dynamic change of the AC system voltage, the difference between the reactive power provided only by the reactive power compensation devices and by the combined modular multilevel converters (MMC) and reactive power compensation devices is analyzed. It is concluded that using MMC to provide a proportion of reactive power enhances the reactive power support to the AC system during faults. Then, the transient voltage model considering the reactive power support of MMC is established, and the critical reactive power consumption of line commutated converter (LCC) is quantified. It is concluded that the reactive power consumption of LCC exceeding its critical value deteriorates the transient voltage. A coordinated support strategy for the sending-end AC system based on reactive power support of MMC and reactive power regulation of LCC is proposed. It can effectively address the challenge of weakened reactive power support to the AC system due to voltage drop, thereby preventing the unbalanced reactive power from deteriorating the transient voltage, and realizing active support of the transient voltage. Finally, a simulation model is established on the PSCAD/EMTDC platform, and the simulation results validate the effectiveness of the proposed strategy in supporting the transient voltage, under different fault types, durations, severities, and locations.