This study proposes a novel wearable device to monitor and record the posture and alignment of spine. The proposed device adopts an under-actuated mechanism design which allows it to adapt to the multiple-degrees-of-freedom spinal posture with minimum weight and complexity. To ensure the validity of measurement and comfort of wearing, the mechanism parameters are determined firstly by considering a special posture then are fine-tuned using an optimization algorithm so that uniform contact forces for several selected spinal postures can be achieved. In the experiments, it is demonstrated that the device can automatically maintain contact with the wearer’s back and offer real-time spinal posture and alignment data for medical diagnosis and treatment.