For the problem of surface roughness and depth of countersink of weakly rigid CFRP/aluminum alloy laminated members, an experimental study on rotary ultrasonic countersinking of laminated members was carried out. The experiments focused on the comparison of countersinking with and without ultrasonic vibration under different rigidity conditions, and revealed the influence of process parameters, ultrasonic field energy and rigidity of hole-making position on axial force and surface roughness of countersinking. The experimental results show that ultrasonic vibration effectively reduces the axial force and improves the wall quality of the countersink, with an average reduction of 12% in axial force and 10% in roughness R_a. The axial force and wall roughness of the countersink in the hole with the lowest transverse stiffness are the highest, while the axial force and wall roughness of the countersink in the rest of the holes increase with the decrease of axial stiffness. Meanwhile, the rotary ultrasonic machining can achieve effective chip breaking of aluminum alloy and avoid secondary damage to CFRP countersink hole wall by long chips.