Voltage instability often follows multi-contingencies or cascading failures， but traditional deterministic methods tend to be conservative in dealing with these scenarios. Risk evaluation method can overcome these short-comings. Risk assessment of static voltage instability is discussed in recent literatures. In this paper， the risk index and assessment method for transient voltage instability are presented. The influencing factors of risk are analyzed. The proposed theory is applied to an actual grid. The relationship among risk， SVC investment and under voltage load shedding（UVLS） scheme is studied. Furthermore， a risk based method for determining SVC optimal investment is proposed.