Continuous measurement of blood pressure based on pulse transit time (PTT) using GMR sensors is the state-of-art non-invasive cuffless method in which modulated magnetic signature of blood (MMSB) is used. In this paper, the mechanism of MMSB is investigated. According to the experimental results, it is found that both blood pulse flowing through the applied magnetic field and the displacement of the GMR sensor caused by blood pulse contribute to the disturbance of magnetic field detected by GMR sensors. The feasibility of MMSB method is discussed as well.