An organic light-emitting display device and a method for driving the same are provided. The organic light-emitting display device analyzes input image data in unit of a window mask to detect a halftone data block, adjusts a voltage corresponding to grayscale 0 of center data disposed at the center of the halftone data block to a voltage higher than 0V, and adjusts the voltage corresponding to grayscale 0 in a data block other than the halftone data block to 0V, such that a data voltage swing width at low grayscales can be reduced so as to prevent voltage drop in pixels, thereby improving picture quality.