In view of transient overvoltage problem of alternating current（AC） system due to complete blocking of after ultra-high voltage direct current（UHVDC） system

it is concluded that surplus reactive power and short circuit capacity are the main factors that cause transient voltage rise.Considering reactive power compensation capacity gain effect and reactive power exchange between AC and DC systems

the calculation formula of reactive power allowance is obtained and the calculation method of transient bus rise is derived.According to the c alculation method of transient voltage

two new indexes are proposed to measure the degree of transient overvoltage after the complete blocking of DC

and the relationship between the blocking capacity and the transient voltage rise is quantitative analyzed.Finally

the CIGRE DC transmission standard test system and Xinjiang Tianzhong DC transmission system are setup respectively on the basis of DIgSILENT simulation platform so to verify the validity and correctness of the calculation method of transient voltage rise.