In view of the unclear mechanism of transient voltage caused by a fault occurred in the near area of HVDC sending system with the interaction of AC and UHVDC, based on the in-depth analysis of the interaction of reactive power AC and DC systems, the dominant factors affecting transient voltage are identified. Considering the fault duration and severity, the expression for calculating the transient voltage of the commutation bus is established, revealing the mechanism of the influence of reactive power consumption of the rectifier on transient voltage under different fault severity and short-circuit ratio. Based on this, an expression for calculating the DC current command value is derived and established, and an optimization strategy for rectifier control is proposed to achieve active support of the DC system for transient voltage. Finally, the simulation results based on PSCAD and CIGRE HVDC have verified the correctness of the theoretical analysis and the effectiveness of the transient voltage support strategy.