Electrocatalytic reduction of water is a promising route to produce hydrogen under mild conditions. High-index facet control is one of the most promising ways to achieve excellent hydrogen evolution reaction (HER)catalytic activity. However, the mechanism of the high-index facet enhancement remains unclear. Here, we combine in situ Raman spectroscopy and theoretical calculation to elucidate the mechanism of HER catalytic performance enhanced by high-index facets on Ti@TiO2 nanosheets. During HER process, water molecules tend to adsorb to the high-index facet surface and then reduce to hydrogen. Our work lays the foundation for future exploration the mechanism of electrocatalysis on transition-metal-based electrocatalysts by in situ Raman spectroscopy in energy and environment-related issues.