A flash memory and an operation method thereof are provided. The flash memory includes a plurality of memory cell strings and a pass voltage generator. Each of the memory cell strings includes a plurality of memory cells. The pass voltage generator is configured to provide a pass voltage to a plurality of word lines of a plurality of unselected memory cells of a selected memory string. During a reading operation, the pass voltage generator raises the pass voltage from a first voltage at a first time point, and raises the pass voltage to a second voltage at a second time point. The second voltage is lower than a target voltage times a preset ratio The first time point is earlier than a start time point of a bit line voltage received by the selected memory cell, and the second time point occurs at the start time point of the bit line voltage.