A SIM card authentication system and a SIM card authentication method adapted for a mobile phone are disclosed. Once the mobile phone is lost, when a SIM card is inserted into the mobile phone, the SIM card authentication system controls the mobile phone to display the user interface, the user must input the right authentication phone number to use the mobile phone. If the user inputs a wrong authentication phone number, the user is illegal and cannot use the mobile phone, the system acquires the phone number of the inserted SIM card to notify a legal user. Therefore, the mobile phone protects and hides stored information from the user, and the legal user may get back the mobile phone according to information from the inserted SIM card.