Recently, the carbonaceous sulfur hydride was synthesized [E. Snider et al., Nature (London) 586, 373 (2020)], and the measured superconducting transition temperature (${T}_{c}$) is about 278 K under 267 GPa, placing it as a promising candidate of the room temperature superconductor. However, the ultrahigh synthesis pressure is indeed a challenge for experimental technique, stimulating us to search other high temperature superconductors under relatively low pressure. Here we perform comprehensively structural searches of ternary alkaline (Am) boron hydrides. Our calculations indicate that the ternary ${\mathrm{AmB}}_{2}{\mathrm{H}}_{8}$ hydrides are energetically stable at about 10 GPa. Interestingly, the metallic $\mathrm{Rb}/{\mathrm{CsB}}_{2}{\mathrm{H}}_{8}$ polyhydrides are superior superconductors with high ${T}_{c}$ of $\ensuremath{\sim}100$ K at 25 GPa. The electron-phonon coupling calculations suggest that the Am atoms are promising electron contributors, which effectively improve the ${T}_{c}$ and decrease the synthesized pressure in complex hydrides. These findings elucidate the superconductivity mechanism of ${\mathrm{AmB}}_{2}{\mathrm{H}}_{8}$ hydrides and offer crucial insights for design and synthesis of novel superconductors.