Liquid Metal (LM) is a large category of emerging functional materials with unique physical and chemical properties such as low melting point, high electrical and thermal conductivity, high boiling point, and low toxicity. In recent years, with the development of flexible electronics and wearable sensor technology, the application of LM in flexible wearable devices for physiological signal monitoring has become a hot research topic, among which gallium (Ga) based alloys are the most widely used liquid metals. This chapter mainly summarizes the basic physical properties of Ga-based liquid metals, such as toxicity, oxidizability, wettability, electrical conductivity, and thermal conductivity. It introduces three material forms of liquid metal flexible electronic devices: original state liquid metal, liquid metal nanoparticles, and liquid metal silicon-based composite materials, and introduces the processing technology of flexible electronic devices based on these three forms of liquid metal materials. Ga-based liquid metal flexible electronic devices can monitor various forms of physiological signals, for example, capacitive/resistive pressure sensors based on Ga-based liquid metal can monitor pulse signals, flexible stretchable strain sensors can monitor limb movements, liquid metal electrodes can monitor blood glucose, etc. At present, liquid metal flexible wearable monitoring devices are still in the preliminary research stage, including the synthesis of liquid metal materials, processing and preparation methods, functional flexible electronic devices, etc., and still have a lot of room for development.