A memory device and an operation method thereof are provided. The operation method includes: in a programming operation, programming a plurality of threshold voltages of a plurality of switches on a plurality of string select lines and a plurality of ground select lines as a first reference threshold voltage, and programming a plurality of threshold voltages of a plurality of dummy memory cells on a plurality of dummy word lines as being gradually increased along a first direction or a second direction, and the threshold voltages of the dummy memory cells being higher than the first reference threshold voltage; wherein the first direction being from the string select lines to a plurality of word lines and the second direction being from the ground select lines to the word lines.