A coupling device for coupling a bone anchor to a rod is provided, the coupling device including a receiving part (5) comprising a first end (5a), a second end (5b), a longitudinal axis (c) extending through the first end and the second end, a channel at the first end for receiving a rod, and an accommodation space (52) for pivotably holding a head (3) of a bone anchor (1)； a pressure member (6) configured to be positioned in the receiving part (5) and to exert pressure onto an inserted head (3) to lock the head in the receiving part, the pressure member comprising a cap-like portion (65) having a pressure exerting surface and a deformable portion (68,89) comprising a free end (68a, 69a), wherein the deformable portion (68, 69) is configured to assume at least a first configuration and a second configuration, and wherein the pressure member (6) is movable along the longitudinal axis from an insertion position to a locking position and wherein the cup-like portion (65) is expandable in the accommodation space (52) in the insertion position； and a locking member (8) configured to be inserted into the channel to move in the channel along the longitudinal axis and to exert a force onto the deformable portion (68,69) of the pressure member； wherein when the head (3) is in the receiving part, and when the deformable portion (68, 69) of the pressure member is in the first configuration and the locking member (8) is in contact with the deformable portion at a first axial position relative to the receiving part, the free end (68a, 69a) of the deformable portion is supported by the receiving part (5) and the locking member (8) is moveable towards the head to exert a force on the deformable portion for adjusting the deformable portion from the first configuration to the second configuration.
