A semiconductor memory device and a method of operating the same are provided. The method of operating the semiconductor memory device includes determining a target word line coupled to an over-programmed memory cell, backing up data stored in memory cells coupled to the target word line in a second memory area, wherein the se second memory area is different from a first memory area where the memory cells coupled to the target word line are disposed, and applying a stepped-up read pass voltage to the target word line when a read operation is performed on a selected memory cell in a memory block coupled to the target word line, wherein the selected memory cell is different from the over-programmed memory cell. Therefore, the operation reliability of the semiconductor memory device is improved.