Prevention of pressure ulcers, which result from persistent mechanical loading of soft tissues, is a common issue among wheelchair users. In the case of deep tissue injury (DTI), it is difficult to predict where tissue breakdown will occur, as it initiates from tissues under the skin. In this paper, a new methodology to assess the risk of DTI using a compliant buttock dummy is proposed. The dummy is composed of pelvis model with six-axis force sensors and silicone rubber covering it to imitate the anatomical form of a human buttock. Six-axis force sensors attached to the pelvis model enable the measurement of external forces and moments during the sitting posture and estimate points of action on bony prominences such as ischial tuberosities by an algorithm. Experiments have been performed to test the accuracy of position estimation by placing point load on the bony prominences model. Next, the relationship between external forces and internal pressure has been confirmed by adhering pressure sensors on the surface of rigid models covered by soft materials. The results suggested the potential of the proposed dummy for the assessment tool of DTI risk.