This paper reviews the recent research progress on key sensing materials for flexible sensors. Particular attention is given to four major categories of materials: metals, hydrogels, carbon-based materials, and conductive polymers. The design strategies, performance characteristics, and application prospects of these materials are systematically analyzed. Metal materials, through dimensional reduction and liquid metal technologies, simultaneously achieve high electrical conductivity and good flexibility. Hydrogels exhibit excellent biocompatibility and diverse conductive mechanisms, enabling broad applications in biosignal monitoring. Carbon-based materials and conductive polymers offer a favorable balance between performance and processability, supporting low-cost and large-scale manufacturing of flexible electronic devices. Despite their respective advantages, these materials generally suffer from limitations such as poor stability, low durability and relatively high cost. Looking ahead, future research is anticipated to emphasise multi-material composites, intelligent structural design and green manufacturing processes, thereby advancing the practical application of these materials in medical monitoring and human-machine interaction.