In this work, a novel plasmonic heterodimer with controllable hot spot was designed and applied to regulate surface plasmon coupling electrochemiluminescence (SPC-ECL) polarization sensing system. The heterodimer nanostructure consisted of individual Au-Ag core-shell nanocubes (Au@Ag NC) and Au nanospheres (Au NS), which were precisely assembled by thiol-DNA and biotin-streptavidin. The asymmetric nanostructure can significantly modulate the ECL intensity and emission polarization angle based on the synergy of the surface plasmon coupling (SPC) effect and the lightning rod effect with extraordinary field enhancement in the hot spot region. As a result, the isotropic ECL signal of zinc-doped nitrogen dots (Zn-N dots) was regulated in the directional emission. Furthermore, the SPC-ECL biosensor was successfully applied to detect miRNA-182 in triple-negative breast cancer (TNBC) tissues. The research on the established relationship between ECL polarization analysis and plasmonic heterodimers can provide a new pathway for the development of ECL sensing platforms.