A clamp body includes a first clamping member and a clamping seat that are integrally formed together, a pivot hole formed in the clamping seat, and a second clamping member pivotally connected to the pivot hole. Also included are an elongated through hole formed in the first clamping member, a circular arc wall extending from the elongated through hole and gradually shallowing from a depth, a position-limiting axle between inside wall surfaces of the second clamping member, a tightness adjusting assembly including a rounded-head pushing rod, and a rotation assembly for coupling the clamp body and the holder. As such, the clamp body that has one arm to fix and one arm to move could improve stability of clamping. The arrangement of the elongated through hole and the circular arc wall gradually shallowing from a depth is used to reduce a size of the tightness adjusting assembly projecting to outside.