Strain sensors with good stability are vital to the development of wearable healthcare monitoring systems. However, the design of strain sensor with both duration stability and environmental stability is still a challenge. In this work, we propose an ultra-stable and washable strain sensor by embedding a coupled composite film of carbon nanotube (CNT) and Ti 3 C 2 T x MXene into polydimethylsiloxane (PDMS) matrix. The composite strain sensor with embedded microstructure and uneven surface makes it conformal to skin, while the CNT/MXene sensing layer exhibits a resistance sensitive to strain. This sensor shows reliable responses at different frequencies and with long-term cycling durability (over 1,000 cycles). Meanwhile, the CNT/MXene/PDMS composite strain sensor provides the advantages of superior anti-interference to temperature change and water washing. The results demonstrate less than 10% resistance changes as the temperature rises from −20 to 80 °C or after sonication in water for 120 min, respectively. The composite sensor is applied to monitor human joint motions, such as bending of finger, wrist and elbow. Moreover, the simultaneous monitoring of the electrocardiogram (ECG) signal and joint movement while riding a sports bicycle is demonstrated, enabling the great potential of the as-fabricated sensor in real-time human healthcare monitoring.