This paper investigates the dynamic characteristics of active power and reactive power during HVDC commutation failure period. The quantitative index for assessing voltage supporting capability of AC system for HVDC system is introduced. The influence of simultaneous commutation failures of Guangdong multiple HVDC on the secure operation of the power grid is analyzed from three aspects: AC area where AC fault may cause multiple HVDC simultaneous commutation failure; multi-infeed effective short circuit ratios and the number of power grid instability under severe AC system failure conditions. Technical countermeasures including HVDC control strategy optimizing, dynamic reactive power compensation installation, AC power grid topology reconfiguration and VSC HVDC utilization are proposed for dealing with voltage instability risk of Guangdong power grid.