Abstract: The transient overvoltage caused by the DC blocking fault of the UHVDC transmission system and the short-circuit fault of the AC system has a serious impact on the stability of the system. Quantitative assessment of the transient performance index has important guiding significance for the dispatching operation of AC/DC power system. This paper analyzes the mechanism of transient overvoltage in AC system, deduces the calculation method of transient overvoltage on the basis of reactive power compensation and short-circuit ratio, and then defines the transient voltage evaluation index R_r. Research shows that, as the index R_r decreases, the transient voltage of the commutation bus at the sending end will increase after a system fault, and commutation failures will also occur at the receiving end. Finally, on the basis of the CIGRE benchmark HVDC standard test system, the effectiveness of the proposed index R_r is verified.