The present invention relates to a system and a method for mobile cross-authentication comprising: generating an online authentication code (Ocode) and a mobile authentication code (Mcode) from an authentication server device when performing online authentication, providing the online authentication code (Ocode) and the mobile authentication code (Mcode) to a computer terminal device and a mobile terminal of the user respectively, receiving and verifying the online authentication code and the mobile authentication code received by the computer terminal device and the mobile terminal to the authentication server device through the mobile terminal and the computer terminal device respectively.