The disclosure provides a NAND flash memory and a reading method thereof, which may read a negative threshold value of a memory cell without using a negative-voltage-generating circuit. The disclosed NAND flash memory includes a sense amplifier, a bit line selecting circuit and an array having a plurality of NAND string units. The disclosed NAND flash memory includes a ΔV supplying portion element that applies a positive voltage to a source line, a P well formed with a selected memory cell, and a non-selected bit line which is adjacent to a selected bit line, within a predetermined time period, after the selected bit line is pre-charged and during a reading process.