A method of operating a semiconductor memory device includes applying a program pulse at least once to each of a plurality of pages; performing a pre-read operation on a reference page among the plurality of pages through an initial test voltage; repeating the pre-read operation by controlling the initial test voltage until a result of the pre-read operation is a pass; setting the initial test voltage of when the result of the pre-read operation is the pass as a reference test voltage; and detecting a defective page among the plurality of pages by performing read operations on the plurality of pages through the reference test voltage.