A method for a successive approximation register ADC which includes at least one capacitor array and a plurality of switches is provided, in which the capacitors of the capacitor array are one-to-one corresponding to the switches. The method includes the following steps: firstly, at least one multiplexer is configured. Then, a first comparison voltage is outputted based on the terminal voltages on the terminals of the capacitor array, and a comparison result is outputted according to the first comparison voltage and a second comparison voltage. Afterwards, a sequence of comparisons is controlled based on the comparison result to enter into a sequence of comparison phases. Finally, the switches are orderly selected, by the multiplexer based on the comparison phases, to switch directly according to the comparison result.