Abstract: Protocol is the specification of the network communication. Then cryptographic protocol, whose safety is based on signature or authentication technology, is one of the key techniques of information security. The technique of signature or authentication needs huge computation during communicating, which brings barriers for many communication devices because of their limited computing power. Therefore, it is an aim of studying information security to design a secure cryptographic protocol, which is practical but doesn't need huge computation. In this paper, a novel key agreement protocol is proposed, which is based on the binary tree and the homomorphic mapping of integer multiplication. Meanwhile, an experiment is carried out in an open source (OpenSSL) systems to test how nodes of leaf binary trees affect network communication and the result of the experiment is analyzed. Our scheme is successful because our key agreement protocol is proved to be safe in the random oracle model. That is to say, in the PKI system, our key agreement protocol meets the requirement of the indistinguishable chosen plaintext attack (IND-CPA ) security. Compared with previous protocols (like MTI, MQV, HMQV), our key agreement protocol has many advantages: the computation is small; only one strong security assumption is needed; it dispenses with extra authentication of MAC and digital signature; and communicating parties can authenticate implicitly through unsafe channels.