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We propose a surface plasmon interferometer based on a wedge metal waveguide. Working principles of the device are introduced through theoretical analyses. The interference performances are investigated by finite-difference time-domain (FDTD) method. Because of the special wedge geometry, the interferometer shows an extremely low coupling loss in comparison with the previous reports. An interesting surface plasmon polariton (SPP) coupling and splitting phenomenon is found at the wedge section. It is then split into two surface plasmon modes in two interference arms. Refractive index sensing applications of this interferometer are also discussed, which shows sensitivity as high as 6 × 10³ nm/RIU.