developing an electrocatalyst with remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic performance remains a critical challenge for achieving large-scale hydrogen production. As one of the most efficient HER catalysts, noble metal platinum (Pt), with its unique d-orbital electron configuration, plays a crucial role in various catalytic reactions, and its compounds are also widely utilized. Here, we investigate high activity catalysts composed of different ratio Pt and C elements by composition-adjustment strategy, four highly stable and metallic monolayers are confirmed, i.e. PtC, Pt2C, Pt3C, and Pt4C. The Pt2C monolayer with low Pt loading exhibits the highest electron occupation at the Fermi level, H2O adsorption and dissociation of more efficient and easier, providing a platform for investigating the properties of electrocatalysis. The HER activity of the Pt2C monolayer is predicted to be superior to that of commercial Pt through several reaction factors, i.e. Gibbs free energy, active site density, and exchange current density, which can primarily be attributed to its Janus steric configuration and excellent conductivity. Moreover, the OER overpotential of the Pt2C monolayer is low to 0.58 V, which is comparable to that of the prevalent RuO2 and IrO2 catalysts. All of the intriguing properties make the Pt2C monolayer a promising bifunctional catalyst toward HER and OER.