Conventional phase-based surface plasmon resonance (SPR) sensing can achieve 10-8 RIU, but commonly requires two-beam interference. It therefore leads to complexity in terms of utilized devices, poor anti-noise ability, and demand for fine working conditions. With these requirements imposed, conventional SPR sensing has difficulties in commercial use. In this Letter, we report a simple, compact, and phase-sensitivity-doubled self-mixing interference (SMI)-based SPR sensing approach. The reported approach employs SMI and, therefore, needs only one optical path, enabling the advantages of compactness and simplicity in experimental setup, and strong anti-vibration property. With the proposed setup, the phase of light from the prism to the sample changes twice. Consequently, the sensitivity of phase is doubled. For experimental demonstration, we monitor the refractive index change of NaCl solution by using the proposed technique. The experimental results coincide with the theoretical analysis and simulation results.