In the AC/DC hybrid system， the AC-system fault is the main cause for the commutation failure of the DC system. From the view of the commutation voltage-time area， this paper reveals the mechanism of the effect of initial fault voltage angle（IFVA） on the commutation failure and indicates that the commutation voltage-time area can be the index for evaluating the risk of commutation failure. Furthermore， under different system structure， parameters， operating and fault conditions， the IFVA has different effects on the commutation failure. Extensive simulations based on the CIGRE HVDC system using PSCAD/EMTDC software are done to verify the theoretic analysis and the commutation failure criteria presented in this paper. The results show that the analysis and criteria are accurate and can be the theoretic basis of researches on AC/DC system interactions， commutation failure avoidance， control system settings in HVDC system and operation characteristic analysis of AC network relaying at the inverter side.