An analog-to-digital conversion unit (ADC unit) and an analog-to-digital converting method (ADC method) are provided. The ADC unit has a plurality of sub analog-to-digital converters and an encoding unit. Each of the employed sub analog-to-digital converters is coupled to two threshold voltages non-successive in terms of levels arrangement, compares the input voltage with the two threshold voltages and outputs two bits according to the comparison results. In this way, the difference between the two threshold voltages coupled by each of the sub analog-to-digital converters can be larger, which is advantageous in advancing the analog-to-digital converting accuracy.