As a classic flexible material, hydrogels show great potential in wearable electronic devices. The application of strain sensors prepared using them in human health monitoring and humanoid robotics is developing rapidly. However, it is still a challenge to fabricate a high-toughness, large-tensile-deformation, strain-sensitive. and human-skin-fit hydrogel with the integration of excellent mechanical properties and high electrical conductivity. In this study, a flexible sensor using a highly strain-sensitive skin-like hydrogel with acrylamide and sodium alginate was designed using liquid metallic gallium as a "reactive" conductive filler. The sensor had a low elastic modulus (30 kPa) similar to that of skin, a high-toughness (2.25 MJ m-3), self-stiffness, a large tensile deformation (1400%), recoverability, and excellent fatigue resistance. Moreover, the addition of gallium might enhance the electrical conductivity (1.9 S m-1) of the hydrogel while maintaining high transparency, and the flexible sensor device constructed from it showed high sensitivity to strain (gauge factor = 4.08) and pressure (gauge factor = 0.455 kPa-1). As a result, the hydrogel sensor could monitor various human motions, including large-scale joint bending and tiny facial expression, breathing, voice recognition, and handwriting. Furthermore, it might even be used for human-computer communication.