By introducing of the natural law of survival of the fittest
an improved method of automatically designing cryptographic protocols is proposed.This method applies SVO logic as a basic tool to describe protocols
constructs a fitness function to assess the performance of individuals systematically and designs a key agreement protocol to meet the corresponding objectives for security.The result shows that: as the generation
the protocol has been verified concurrently when it is generated
to ensure its correctness and security.So the method has a strong feasibility and applicability.