Abstract: Due to the shallow burial depth and complex geological conditions of coal seams in some open-pit coal mines, coal seams are often exposed and in full contact with air, leading to spontaneous combustion and the formation of burnt rock. Burnt rock contains abundant micropores, which allow rainfall and groundwater to infiltrate, forming water-rich zones that pose significant threats to safe production in open-pit coal mines. To prevent water-rich burnt rock zones from breaching slope boundaries and entering the open-pit mine, thereby affecting normal production, this study proposes an optimization method for the width of water-isolating coal-rock pillars in irregular burnt rock areas of open-pit coal mines. For dynamically irregular water-rich burnt rock zones, the irregular boundary between burnt rock and coal-rock regions is extracted from geological cross-sections. Multiple mathematical functions are used to analyze the irregular boundary lines. When a single fitting does not meet accuracy requirements, segmented fitting with multiple functions is applied. The segmented fitting ranges are determined based on the fitting degree, and segmented boundary functions are obtained. The minimum width of coal-rock pillars is then calculated by combining the segmented fitting functions with the minimum width formula for water-isolating coal-rock pillars. By jointly solving the function values and calculated results, the final minimum width of coal-rock pillars for each stratigraphic section is determined, and the maximum value is selected as the ultimate minimum width for a given profile. A case study of a typical cross-section in the Daan Lake No. 2 Open-Pit Coal Mine was conducted. Given that the stability factor of the aquifer slope is known and the slope is in a stable state, segmented fitting was performed using Fourier and Gaussian functions. The fitting degrees were 0.999 6 and 0.999 9, respectively, with residuals generally controlled within 0.6 m and 0.7 m. The final minimum width of the water-isolating coal-rock pillar for the profile was determined to be 100 m.