Disclosed are a client, a server, an RADIUS capability negotiation method and system, and the method includes: a client transmits to a server a first message carrying RADIUS capability parameters of the client; the server captures the RADIUS capability parameters in the first message, matches the RADIUS capability parameters in the first message with RADIUS capability parameters of the server to obtain a matching result, and transmits the matching result to the client through a second message; and the client determines whether to establish effective communication with the server according to the matching result in the second message. By means of the technical solutions of the disclosure, it is possible to extend the RADIUS protocol, and to solve the problem existing in the current RADIUS protocol that both sides in communication cannot perform RADIUS capability negotiation.