Several improved short circuit ratio（SCR） indices have been proposed for multi-infeed-HVDC power system， however studies on their feasibility are insufficient and non-quantitative. In this paper， the South China power grid with 5-HVDC-link is applied as the prototype for simulation test and the performance of two improved short-circuit ratio indices MSCR and MIESCR are thoroughly evaluated. Their features as well as problems existing in applications to the AC-DC parallel power networks are analyzed. A novel structure-related index， i.e. the transient voltage supporting index（TVSI） is proposed to evaluate the interaction strength of the HVDC terminals. Comparisons on the TVSI and multi-infeed interaction factor， as well as tests on the relationship of TVSI with the inverter-bus transient voltage sag and commutation failure lasting-time all prove its feasibility and good performance.