Mobile ad-hoc networks (MANET) have great martial value and wide business application prospect. In MANET, owing to many new characteristics such as central-less control and multi-hop communication, the security situation is more rigorous than that in the traditional network. Especially, when the number of nodes increase, the difficulty of building network, network avalability and network security will be influenced badly. In this paper the current state of MANET research is first surveyed and then the state of the key technologies involved with large scale MANET security, such as security model and scheme, secure clustering, group key management and so on, is discussed in detail. Finally, the main research directions of large scale MANET security technology are presented, which are the security networking technology, the MANET protocol security proving technology, the key management technology, the model simulation and security evaluation technology.