Prevention of pressure ulcer (PU) has been hampered for a long time by limitations of present risk assessment methodologies for carers. This paper introduces a novel solution of a portable bioimpedance spectrometer (BIS), which is designed to detect in vivo subtle changes in electrical properties of tissues at the portentous onset of PU and thereby establish a reliable assessment method for early detection of PU. The principle of the modified tetrapolar method applied in BIS and system designing methodology are explained thoroughly in this paper. The BIS system can measure complex impedance of human tissues over a wide range of frequencies from 10Hz to 1MHz based on direct digital synthesis (DDS) technique. Different from conventional bioimpedance measuring systems based on phase sensitive detection, the new BIS system shows its integrity in theoretical analysis and simplicity in practical implementation. As an objective, fast, straightforward, and inexpensive technique that captures fundamentally different information than subjective assessment and optical judgment, the new BIS may bring to clinical use for risk assessment of PU in the future.