Rationally constructed new catalyst can promote carbon dioxide reduction reaction (CO2RR) to valuable carbonaceous fuels such as formate and CO, providing a promising strategy for low CO2 emissions. Herein, the synthesized Ni3S2@C as a highly efficient electro-catalyst exhibits remarkable selectivity for formate with 73.9% faradaic efficiency (FE) at −0.7 V vs. RHE. At high applied potential, it shows a high syngas evolution with CO/H2 ratios (0.54–3.15) that are suitable for typical downstream thermochemical reactions. The experimental and theoretical analyses demonstrate that the electron-rich Ni2+ in Ni3S2 enhances the adsorption behavior of ∗OCHO intermediate, reduces the energy barrier of the formation of intermediates, and improves the selectivity of the formate product. Attenuated total reflection surface-enhanced infrared absorption spectra conducted in situ show that ∗OCHO intermediate is more likely to be generated and adsorbed on Ni3S2, enhancing the selectivity and activity of the formate product.