A key distributing method, a public key of key distribution centre online updating method, a key distribution centre, a communication entity and a key management system. The system includes: communication entities, a carrying device, a key distribution centre and a database, wherein the carrying device carries or transports the information during the key distributing course and the public key online updating course, the database stores whether each communication entity registered secret service; the database connects with the key distribution centre, the key distribution centre connects with the carrying device, and the carrying device connects with each communication entity. Using the cipher technology of public key, a key distribution system is provided based on principle of three-element peer authentication (TePA). The system safely distributes the communication key to each pair entities to enable keys have PFS attribute, reduces the key management complexity of the system, and realizes online updating of the public key of the trusted third party i.e. key distribution centre.