Provided are a system and method for updating a user identifier (ID). The user ID updating method includes: (a) collecting unauthorized access attempt information for a user ID; (b) creating a user ID update policy for an encoded user ID obtained by encoding the user ID, according to the unauthorized access attempt information collected in operation (a); (c) storing the user ID update policy created in operation (b); (d) loading the user ID update policy stored in operation (c) and determining whether or not to update the user ID; and (e) creating a new user ID if it is determined in operation (d) that the user ID should be updated, and changing the user ID to the new user ID. Therefore, it is possible to ensure security for user IDs, by dynamically creating and updating user IDs according to security environments.