Focused on the shortcomings of traditional transmission reliability margin（TRM） assessment method， a novel concept of TRM virtual insurance is put forward based on the generalized insurance theory. By application of this concept， the optimum decision-making problem of TRM assessment is reconstructed into an optimum decision-making problem of virtual insurance purchasing for constructing the model to analyze. On the basis that the generalized measurements of reliability and economy are realized by generalized insurance theory， the utility function is adopted to describe the reliability preference of decision-makers indirectly. The insurance assets choice theory is applied to establish the decision-making model. Moreover， uncertainty is considered by using Monte-Carlo simulation to evaluate the model parameters. A case study of the IEEE 30-bus system verifies the effectiveness and advantages of the model proposed.