A bearing dismounting device includes a body, a rod, several clamping arms, a tightening element, and an adjusting element. Each of the clamping arms is pivotally installed around the body. The center of the body has a through hole for the rod to mount therein. One end of the rod has a positioning part, and its other end has an operating part. Each of the clamping arms extends a buckling part. The central position on the outer side of each of the clamping arms has an accommodating part. The tightening element surrounds the accommodating parts of the clamping arms. Both ends of the tightening element have an adjusting element, respectively, for urging against the tightening element, thereby positioning the clamping arms.