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High superconducting critical temperature ( ${T}_{c}$ ) of 73.6 K at ambient pressure is predicted in BSiC _2 by means of the first-principles electron–phonon calculations. Without the need for doping or pressurization, the stoichiometric BSiC _2 exhibits strong electron–phonon coupling (EPC) and highest ${T}_{c}$ among BCS-superconductors at ambient pressure. Also, it is hydrogen-free that makes easier the sample growth. The dramatic softening of the ${E}_{2g}$ mode is the main account for the strong EPC. In addition, we find that BC _3 , derived from the replacement of Si in BSiC _2 by another C, is also superconducting with a high ${T}_{c}$ ∼ 40 K. Our work indicates that these materials form a new family of high- ${T}_{c}$ BCS-superconductors at ambient pressure.