Conductive polymer composites (CPCs), with the merits of light weight, low-cost and good processability, have been applied as remarkable sensing materials for their response to various external stimuli, such as strain, pressure, presence of volatile organic liquids. However, widespread application of conventional bulk CPCs as sensing materials is limited due to their relatively low sensitivity and clumsiness in weak stimulus detection. Herein, we propose a simple and cost-efficient strategy to fabricate highly sensitive strain sensors based on ultrathin CPC layer-coated PU yarn via layer-by-layer (LBL) assembly. The resultant CPC@PU yarn strain sensor exhibited desirable sensitivity with a gauge factor (GF) of 39 and detection limit of 0.1% strain as well as good reproducibility over 10,000 cycles. Benefiting from the high sensitivity, our CPC@PU yarn strain sensor was qualified for tiny motion monitoring, including speech recognition, pulse monitoring, and expression detection. Human physiological activities (e.g. swallowing, coughing, chewing, raising head, lowering head) could also be monitored using this CPC@PU sensor. Furthermore, this CPC@PU strain-sensitive yarn could be woven into textiles to product functional electronic fabrics. The high sensitivity of our sensor, together with its significant advantages of low-cost, easy fabrication, washing durability, and environmental friendliness, makes it promising in fabricating cheap and sensitive electronic sensing devices.