A signal processing system includes an adjustable filter, a signal difference calculating unit, a determining unit and a selecting unit. The adjustable filter is configured to receive a digital signal outputted from an analog-to-digital converter, and filter the digital signal to generate a filtered signal. The signal difference calculating unit is configured to calculate and output a signal difference value between the digital signal and the filtered signal. The determining unit is configured to compare the signal difference value with a predetermined threshold value, to generate a comparison result, and generate a selection signal and a bandwidth adjustment signal according to the comparison result. The selecting unit is configured to receive the digital signal and the filtered signal, and output one of the digital signal and the filtered signal according to the selection signal. The bandwidth of the adjustable filter can be adjusted according to the bandwidth adjustment signal.