Mobile sign-in system and method

A mobile sign-in system includes a mobile sign-in server, a sound generating device and a portable electronic device. The mobile sign-in server generates and sends a one-time password and audio control data to the sound generating device. The sound generating device sends out a first sound signal according to the audio comparison data and each character of the one-time password. After receiving the first sound signal, the portable electronic device generates and sends a password to be verified to the mobile sign-in server based on the first sound signal and the audio comparison data. When the mobile sign-in server determines that the password to be verified matches the one-time password, the mobile sign-in server generates a mobile sign-in record containing the identification data of the corresponding portable electronic device.