At present, graphene has been widely used in flexible sensors, however, it is still a problem to prepare graphene-based sensors by a simple and low-cost method. In this paper, graphene nanoplatelets (GNPs) are prepared by thermal expansion method, and a capacitance plate is prepared by sealing a GNPs sheet with polyurethane (PU) medical film. Then two capacitor plates are overlapped each other, and the two sides are sealed with a transparent tape to form a GNPs capacitance sensor. In comparison with existing graphene-based sensors, the fabrication method of the GNPs capacitance sensor is simple and low-cost. What is more, the physiological signals can be measured effectively by the GNPs capacitance sensor. The sensor has been used to measure the signal of finger bending motion and pulse wave signal. The results show that the GNPs capacitance sensor is a promising sensor for effective measurement of physiological signals.