Due to the influence of ore-rock endowment and blasting construction quality, variable cross-section pillars are commonly present in room-and-pillar underground mining mines. To study the bearing capacity effect of variable cross-section pillars, taking the variable cross-section pillar of a limestone mine as the research object, a mechanical model of the variable cross-section pillar was established, the expressions for its stress and safety factor were derived, the relationships between stress, safety factor and factors such as the diameter d of the variable cross-section pillar and the radius R of the variable cross-section area were studied respectively, and the stress distribution characteristic was analyzed. The results show that the stress of variable cross-section pillars changes gradually in the variable cross-section area, and the stress amplitude decreases with the increase of R or d. The safety factor increases with the increase of R, and the rate of change increases gradually. It also increases synchronously with the increase of d. Compared with constant cross-section pillars, variable cross-section pillars have a more uniform stress distribution, a better stress transfer effect at the junction with the roof, a higher safety factor, and better stability. Numerical simulation and engineering examples have verified the reliability of the theoretical calculation results.