An analog-to-digital converting system and a method with offset correction mechanisms are provided. The method includes steps of: obtaining a direct current offset of an output voltage of a digital analog conversion unit in a system; obtaining first capacitance weights and second capacitance weights sequentially from small to large; subtracting the direct current offset from a digital signal; and multiplying bit values of the digital signal respectively by the corresponding first capacitance weight value or second capacitance weight value to output a decode signal.