The discovery of metal borides with higher transition temperature ($T_{\mathrm{c}}$) has been receiving continuous attention since the observation of superconductivity in ${\mathrm{MgB}}_2$ at 39 K. Here we propose a rational design strategy for high-$T_{\mathrm{c}}$ ternary metal borides through the cosubstitution of Mg with two compensating metal elements that possess an effective isovalency of 2. Guided by this principle, we theoretically predict a collection of ternary boride superconductors, including a representative example of ${\mathrm{KAlB}}_4$ that has an isostructure to ${\mathrm{MgB}}_2$, with $T_{\mathrm{c}}$ up to 67 K at ambient pressure. Our detailed analysis reveals that, compared with ${\mathrm{MgB}}_2$, due to the Stark effect caused by the difference in valence of two metal ions, the $\sigma$ bands of B layers get significant splitting and exhibit more flat character, resulting in enhanced electronic occupation; furthermore, the softened in-plane $E_{2\mathrm{g}}$ modes produce larger deformation potential and electron-phonon coupling with B $\sigma$ states, which in turn results in larger superconducting gaps and much higher $T_{\mathrm{c}}$ value in ${\mathrm{KAlB}}_4$. We further propose a feasible synthesis route of ${\mathrm{KAlB}}_4$ to stimulate experimental progress. Our approach paves the way for finding more high-$T_{\mathrm{c}}$ ternary boride superconductors at ambient pressure.

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• Electron-phonon coupling
• Electronic structure
• Flat bands
• Multiband superconductivity
• Stark effect
• Borides