Unidirectional excitation of surface plasmon polaritons has been applied widely in optical communication, integrated optics, lithography and so on. A subwavelength structure of metallic slit-groove in theory is proposed. This structure achieves unidirectional excitation of surface plasmon polaritons along the metallic film under back illumination. During our design, finite-difference time-domain method (FDTD) is adopted for numerical simulation. At first, the electric field intensity of surface plasmon polaritons which transmits through the groove can reach minimum by changing the depth and width of the groove, and the physical mechanism can be explained with the scattering matrix theory very well. Then based on surface plasmons′ interference principle, the interference of surface plasmon polaritons, which propagate along the side deviating from the groove, can be strengthened by changing the distance between the slit and groove. Thereby, the metallic slit-groove structure realizes unidirectional excitation of surface plasmon polaritons. The maximum splitting ratio can reach up to 8.