Recently, applying nanoscale functional materials, there have been great advances in the flexible sensor system, which provides a large number of applications for soft electronics, such as skin-attachable sensors, artificial electronic skins, and soft robotic systems. Here, we developed a highly sensitive and flexible device on the basis of polymeric piezoelectric nanofibers and elastomeric packing structures. To produce the nanofibers, we applied the electrospinning process with a representative piezoelectric co-polymer, poly (vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE). Unlike the conventional electrospinning, we applied an anisotropic fiber collection system, which could obtain uniaxially aligned nanofiber array. The aligned nanofibers were sandwich-packed with bridge-shaped PDMS substrates, thereby integrating the flexible piezoelectric sensor. As an external force made a deflection of the bridge in the sensor, the embedded nanofibers generated piezoelectricity in a longitudinal direction of the fibers. The piezoelectric sensor generated good discernable outputs versus the varied mechanical input deflection from tens of micrometers to the sub-micrometer. With this great sensing ability, we could monitor heart pulse signals on the wrist skin by measuring tiny deflections generated from the expansion of the radial artery underneath the skin. Our study suggests a potential application of flexible sensor in the field of wearable health-monitoring medical systems.