The invention discloses an instant messaging method and system for user mode encryption. A user controllable encryption technology is used for protecting the instant messaging information of a user, and the personal privacy of the user can be effectively protected. The principle of the system is to construct a set of user unique feature password, and to encrypt the messaging information of the user based on the password, thereby guaranteeing that only the lawful owner of the password can decrypt the information. The specific processes are as follows: (1) initializing; (2) message encrypting; (3) message transmitting; and (4) message decrypting. The system comprises a server-side and a plurality of clients, wherein the clients are connected with the server-side through the network. The system has the advantages that the security is high, the information content transmitted by the user cannot be obtained by a messaging server, the efficiency is high, and the messaging efficiency consistent with the similar unencrypted product is provided.