A four-layer D-shaped optical fiber surface plasmon resonance (SPR) for high-sensitivity polarimetric strain sensor is proposed. In contrast to existing SPR-based sensors, which are based on changes in the refractive index of the over-layer, the sensor proposed in this study is based on the change in the refractive index of the fiber core in response to the application of an axial load. Specifically, the phase difference between the P- and S-waves after passing through the sensor under SPR conditions is measured using a common-path heterodyne interferometer and is used to determine the corresponding change in the refractive index of the core. The experimental results show that the sensitivity of the proposed sensor is around 2.19×104 Deg./ε, i.e., degree/strain. By contrast, that of a conventional (non-SPR) polarimetric fiber sensor is just 5.2×102 Deg./ε.