In order to comprehensively and effectively assess the vulnerability of power communication network, which is an important supporting network of smart gird, a cross-layer assessment method based on the information entropy is proposed. Firstly, a calculation method of power business importance is presented and the business importance is taken as a parameter to model power communication network on the business layer. The importance of edges on the business layer is described by the Edge Business Importance (EBI). Secondly, taking into account the business layer, transport layer and physical layer, Edge Cross-layer Importance (ECI) is proposed and the information entropy of ECI on a network, which is called Edge Cross-layer Entropy (ECE), is defined as the assessment index of network vulnerability. Finally, taking a real communication network as the simulating background, the validity of the method is proved by comparing the change of network vulnerability curves and ECEs under different routing strategies. The proposed method is suitable for not only power communication networks but also all the networks which carry non-uniform businesses.