Wiretap Channel model was first proposed by Wyner in 1974 and has become the basis of secure communication technologies at physical layer. Compared with the public key cryptography, which is almost developed synchronously, it only depends on the assumptions of physical channel rather than computational assumptions. Abundant work in this area has been developed, but there are only heuristic results for security definitions and evaluation methods until 2012. This paper provides a survey of the wiretap channel model, focuses on formal security definitions and keyless secure schemes of it.