Negative pressure (NP) therapy can effectively manage irregular extremity lymphedema but requires significant physical therapist effort. Considering the repeated work during the therapy, using the NP therapeutic robotic systems is recommended to help the therapist decrease the workload. The therapist could make the rapid suction to deform the head lip with the dexterous hands, while the robot fails to feel the interaction during the therapy due to the lost sensors. Therefore, this paper proposes a novel suction head with a variable-sized soft lip for haptic perception. The design of the soft lip is optimised from the previous work, and the detection sensor with strain gauges (SGs) is designed to be located under the lip for haptic perception. Then, the SG voltage was calibrated to the force with the standard weights, and the lip pressure was compensated to the SG voltage in the later experiment. The haptic experiment was conducted when the head contacted the phantom surface by using the manipulator. The results illustrated that the head could detect the contact pose through the distributed forces from SGs during the robot rotation.