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Abstract When highwall mining technology is applied to recover large amounts of residual coal left under the highwall of a big open-pit mine, a reasonable coal pillar width is required to ensure the stability of the web pillars. Using numerical simulations, this paper studied the characteristics of the abutment stress distribution in the web pillars under different slope angles and mining depths, and established a relation describing the stress distribution in the web pillar. The relationship between the abutment stress and the ultimate strength of the web pillar under different pillar widths was also analyzed. In combination with the failure characteristics of the pillar yield zone, this relationship was used to explore the instability mechanism of web pillars. Finally, the optimal retaining widths of the web pillars were determined. Based on the modeling results, a mechanical bearing model of the web pillar was established and a cusp catastrophe model of pillar-overburden was constructed. Additionally, the web pillar instability criterion was derived. By analyzing the ultimate strength of the web pillars, a formula for calculating the yield zone width either side of the pillars was established. Using the instability criterion of web pillars in highwall mining, a reasonable pillar width can be deduced theoretically, providing significant guidance on the application of highwall mining technology.