Different from traditional rigid sensors based on silicon substrates, flexible sensors are suitable for irregularly shaped surfaces and complex measurement situations that need to be embedded in curved surfaces. Due to high flexibility and stretchability, flexible sensors are extensively served as personalized health surveillance and implantable biomedical devices. This paper reviews the latest breakthrough in flexible electronics, including new materials, fabrication strategies, and properties, emphasizing various applications of flexible sensors based on different material designs in health monitoring systems. In addition, the simultaneous transmission and optimization of signals from the human body to the detector, excellent stretchability, transparency, and biocompatibility give attractive prospects for wearable electronics with functions that infinitely tend to resemble real human skin or other skin. Finally, we discuss challenges the flexible sensing systems must rise to and look forward to the development prospect. With the rapid development in the field of flexible strain sensors, the practicality of their large-scale manufacture with a certain commercial and practical value is becoming a trend and an urgent need.