Recently, structure analysis of power grid security using complex network theory has become a hot research at home and abroad. According to the specific characteristics of transmission network, an improved complex network model is constructed because the current complex network model is over simple. Four evaluation indexes, which are transmission network efficiency, transmission network capability, transmission network path redundancy and node topology entropy of the power network, are proposed from different angles. The first three indexes are used to evaluate the overall transmission performance, and the last one is to estimate the bus node importance balance. Based on the above indices, entropy weight method is applied to comprehensive evaluation of the overall security of the complex structure of transmission grids. Simulation results of typical transmission network structure and IEEE typical network model verify the effectiveness of the proposed indexes and method.