Flexible yarn sensors designed for integration into textiles have the potential to revolutionize wearable technology by continuously monitoring biomechanical strain. However, existing yarn-shaped sensors rely on capacitance as a strain-dependent electrical signal and often face limitations in achieving high sensitivity, especially across a broad strain range. Here, we propose a waterproof all-in-one capacitive yarn sensor (ACYS) that is tailored to monitor a wide range of biophysical strains. Owing to the coaxial helical electrode and the ionic liquid-doped dielectric layer, the ACYS demonstrates remarkable stretchability, ultrahigh capacitance variation, and an outstanding gauge factor of 6.46 at 140% strain. With exceptional mechanical durability based on enduring 3300 stretching cycles and favorable resistance to sweat erosion, this 1D structure can be seamlessly integrated into textiles, making it ideal for use in wearable electronics. Demonstrating its application versatility, the ACYS accurately measures biomechanical strain in joint movements, facial expressions, and physiological assessments, making it a promising advancement in wearable technology.