Water splitting is considered as the most promising way to produce hydrogen and develop sustainable energy economy. However, the existence of overpotential still increases the energy costs needed for hydrogen evolution reaction (HER). Recently, many researchers have been devoted to preparing efficient electrocatalysts for HER to lower the overpotential and reduce the energy waste. But from the reaction system perspective, we want to find a more practical and universal way to solve the problem by studying the reaction mechanism and then adjusting the preferred reaction pathways. The adsorption of hydrogen atom on the catalyst surface is the beginning and necessary step for the whole HER electrocatalyst reaction. After that, there are two ways for the adsorbed hydrogen to get another hydrogen atom and form a hydrogen molecule which can be released. One is to combine the neighbor adsorbate hydrogen and the other is to grab a proton from the surrounding electrolyte. The two are competences, and which one is preferred is determined by the reaction energy barrier. If we can evaluate the effect of some factors, like coverage of the adsorbates and surrounding ions, to the energy barrier, there is a possibility for us to design an optimized pathway to improve the energy efficiency, and even get final product selectively. The DFT calculation in different reaction systems needs long-time calculations, so strong computing resources are essential for us.