A kind of link analysis sorting method is applied to power system security assessment. A critical node identification method combined with power tracing technique is proposed. Based on result of the power trace, a new index of system node link strength and evaluation of load importance is defined. Power grid weighted directed graph is established by utilizing system node link strength with system actual power flow direction. Inspired by the idea of Pagerank Algorithm based on link analysis, the computational model of node importance evaluation based on power grid weighted directed graph is established and the critical node effective identification is realized. The proposed identification method bases on the system actual grid topology and power flow distribution, from the view of the global grid energy transmission to identify the key node of system safe operation. The proposed method is verified by taking IEEE-39 system as an example and compared with the existing method. The verification result indicates that this method is more consistent with the system actual operation state. This work is supported by Fundamental Research Funds for the Central Universities (No. 13MS108).