Display omitted •A flexible and sensitive strain sensor was fabricated using biomass Juncus effusus.•Strong flexibility, high sensitivity, and multifunctionality were achieved simultaneously.•Geometric changes and cracks propagation of microfibrils contributed to the performance.•Good repellency against mechanical deformations and chemicals were obtained.•Morse code communication and real-time monitoring of human activities were achieved. Flexible strain sensors have received extensive attention owing to their booming development. However, achieving high sensitivity and stretchability simultaneously for strain sensors is still a challenge. Herein, we report a fiber-based stretchable strain sensor with a unique three-dimensional (3D) reticular structure for human motion monitoring. The biomass Juncus effusus (JE) fiber was functionalized with carbon nanotubes (CNTs) and Ecoflex (EF) using a facile dip-coating-drying approach, and a syringe extraction method, respectively. The resulted JE/CNTs/EF fiber strain sensors exhibited advanced performance of strong flexibility, high sensitivity, and multifunctionality, which displayed a superior sensitivity (gauge factor of 24.95–76.79) and high stretchability (600%). Moreover, good repellency to chemicals and stable responses to various stimuli frequencies over 270 cycles were obtained for the as-prepared sensors. The microfibrils of the JE fiber formed a complicated 3D reticular structure acting as the conductive bridges when stretching. Consequently, the fiber-based JE/CNTs/EF strain sensor can not only accurately detect a full range of body motions and subtle signals, but also can be a promising candidate as a Morse code generator for cryptographic communication.