Flexible strain sensors have become a key component of intelligent wearable electronics. However, the fabrication of strain sensors with wide workable strain ranges and high sensitivity remains a great challenge. Additionally, the rapid development of polymer composites based strain sensors has produced a large amount of e-waste. Therefore, the development of strain sensors with wide strain sensing ranges and high sensitivity based on degradable materials is necessary. In this work, a silicone blocked polyurethane (Si-BPU) with high stretchability and degradability was synthesized and composited with carbon nanotubes (CNTs) to fabricate fibrous strain sensors. The synthesized 0.5% Si-BPU exhibited good biodegradability with a weight loss of 16.47% in 42 days. The Si-BPU/12CNTs fiber based strain sensor achieved a sensing range of 0%–353.3% strain, gauge factor (GF) of 206.3 at 250% strain and of 4,513.2 at 353.3% strain, and reliable stability under 10,000 repeated stretching–releasing cycles. Moreover, the Si-BPU/12CNTs strain sensor showed rapid response time (< 163 ms) and was capable of monitoring various human body movements (elbow bending, finger bending, breath, and swallow). In consequence, this work provides a new and effective strategy for the development of sustainable wearable electronic devices.