A structure of a base drum counter-hoop mounted clamp is provided for attaching a small-sized music instrument to a counter hoop of a base drum. A clamp body is shaped to have upper and lower locking platforms that are substantially parallel and connected by a vertical plate, which has a side surface from which a pivot seat projects to pivotally couple a connection member. Opposite ends of the connection member are respectively coupled to a V-shaped clip body and a threaded bolt bar. With the threaded bolt bar being tightened and driven downward, the clip body is upward pushed against and secured to the counter hoop of the base drum. A holder formed atop the upper platform receives and fixes a support bar that supports an additional music instrument thereto.