A filter and a filtering method are provided. The filter includes a first compare voltage generation unit, a second compare voltage generation unit, a comparator and a first inverter. The first compare voltage generation unit generates a first compare voltage according to an input signal. The second compare voltage generation unit generates a second compare voltage. When the first compare voltage is not over the first reference voltage, the second compare voltage equals the first reference voltage. When the first compare voltage is over the first reference voltage, the second compare voltage equals the second reference voltage. The first reference voltage and the second reference voltage depend on a minimum pulse width. The comparator outputs a filtered signal according to the first compare voltage and the second compare voltage. The first inverter inverts a filtered signal to an output signal.