A memory device comprises a memory cell array, a first and a second pre-charging switch circuits, a selecting circuit, an auxiliary memory cell array, a dynamic voltage controller and a sense amplifier. The auxiliary memory cell array comprises an auxiliary read bit line and a plurality of memory cells arranged in a column and electrically connected to the auxiliary read bit line. The second pre-charging switch circuit determines whether or not to supply a reference voltage to each of the aforementioned memory cells according to a pre-charging control signal. The dynamic voltage controller determines whether or not to supply a voltage to the auxiliary read bit line according to the voltage level of the output signal of the selecting circuit. The sense amplifier compares the voltage levels of the output signal of the selecting circuit and the voltage on the auxiliary read bit line to output a sensing result accordingly.