High-efficiency　and　direction-controllable　launching　of　surface　plasmon　polaritons　(SPPs)　is　highly　desired　in　plasmonic　devices　and　photonic　circuits.　Here,　a　heteroplasmonic　device　is　reported　consisting　of　an　array　of　asymmetric　nanocavities　with　each　nanocavity　constructed　by　an　asymmetric　Ag　nanoshell　(AgNS)　and　a　planar　gold　film　(AuFilm).　A　photoresist　grating　is　used　both　as　a　template　for　fabricating　the　AgNS　array　and　as　the　dielectric　filling　medium　of　the　nanocavities.　Interaction　between　localized　surface　plasmon　resonance　in　the　asymmetric　AgNSs　and　SPPs　in　the　AuFilm　has　enhanced　the　propagating　surface　modes　along　the　gold　film.　A　phase　modulation　induced　by　the　asymmetry　and　interference　between　the　SPP　modes　have　been　the　main　physics　supporting　the　directional　propagation　of　the　surface　waves　or　the　in-plane　optical　diode　effect　at　normal　incidence.　The　SPP　diode　works　in　different　directions　at　different　wavelengths,　which　can　be　controlled　by　adjusting　the　asymmetry　of　the　AgNSs.