A light access authentication method and system, the method includes: the trustful third party writes the MSG cipher text formed by enciphering MSG into the first entity; the second entity attains the MSG cipher text from the first entity, and attains the key from the trustful third party after attaining the MSG cipher text; the MSG cipher text is deciphered according to the key, and the MSG plaintext is attained. The embodiment of the present invention can be widely applied at a condition limited by the equipment and environment, and the access authentication is simplified and lightened.