A modem (10) includes a signal tuner module (101), a signal processing module (103), and a mode switching module (105). The signal tuner module receives a radio frequency (RF) signal in a first operation mode, and converts the RF signal to an intermediate frequency (IF) signal according to the first operation mode. The signal processing module processes the IF signal to generate an error rate according to the first operation mode. The mode switching module determines whether the first operation mode is right according to the error rate, and switches from the first operation mode to a second operation mode when the first operation mode is not right. A method for switching operation modes thereof is also provided.