Flexible tubular strain sensors are crucial for wearable electronics and soft robotics. However, traditional tubular strain sensors are limited by their intrinsic positive Poisson's ratio (radial contraction) during axial stretching, which densifies the conductive network and fundamentally restricts the enhancement of gauge factor (GF). To overcome this bottleneck, we propose a high-sensitivity tubular flexible strain sensor by integrating an auxetic re-entrant hexagonal skeleton with an EcoflexCarbon Nanotube (CNT) composite conductive layer. Finite element analysis (FEA) was employed to systematically investigate the structureproperty relationships. The simulation results reveal that increasing the reentrant angle and strut width effectively amplifies the local strain distribution, thereby enhancing sensitivity. Furthermore, a kinematic interference mechanism was identified: under a fixed tube diameter constraint, increasing the circumferential cell density causes the GF to initially rise and subsequently decline due to structural overlapping during deformation. Conversely, unconstrained structural expansion yields monotonic GF enhancements. Guided by these structural deformation mechanisms, an optimized sensor was fabricated (re-entrant angle 40°, strut width 1.5 mm, 6-cell circumferential array). Electromechanical testing confirms that the auxetic skeleton successfully converts axial straininduced radial contraction into radial expansion. The sensor also demonstrates outstanding cyclic repeatability over 5500 stretching cycles with negligible resistance drift, indicating reliable long-term stability. Within a practical sensing range of 0~15% strain, the optimized sensor exhibits excellent linearity (R² = 0.9850) and achieves a superior GF of 19.50. This represents a 3.13-fold improvement over a conventional hollow tube (GF = 6.23) without the auxetic framework. This study demonstrates that engineering auxetic metamaterials effectively transcends the intrinsic sensitivity limits of traditional elastomeric materials, providing a robust structural design strategy for high-performance tubular flexible sensors.