As a typical disaster-causing geological structure, karst collapse pillar (KCP) is widely distributed in coalfields of northern China. The interior of KCP is filled with loose and weakly cemented rock masses. Fine particles can be eroded under the hydraulic pressure and the disturbance of the coal mining operation. Then, water inrush pathway can be formed easily, resulting in water inrush hazard. The release of untreated coal mine water can pollute the environment and waste the limited water resource in China. To investigate the particle erosion effect on the water inrush mechanism of KCP, FLAC3D numerical investigations were conducted to simulate the water flow process of KCP in the mining floor during the coal seam excavation, according to the stress-seepage coupling model with the consideration of the particle erosion. Besides, the evolution of shear stress field, seepage field, and plastic zone along was obtained as working plane advances. Meanwhile, the influence of the thickness of a waterproof rock floor and the hydraulic pressure of aquifer on the formation of water inrush pathway was analyzed. Numerical results indicated that: (1) Shear failure of the KCP near the side of the working plane occurs under the effect of mining excavation; then, the KCP connects with the damage area around the working plane; finally, the water inrush pathway is formed. (2) Water inrush disaster will not occur immediately when the KCP is connected with the damaged area around the working plane; it only occurs when the KCP is completely exposed in the mining. (3) With the mining advances, the thinner the waterproof rock floor and the greater the hydraulic pressure of the aquifer, the easier the groundwater can lead up, and the KCP tends to be damaged with the formation of water inrush pathway.