To understand the nitrate reduction process on the (101) surface of anatase TiO2-x, two surface models with and without oxygen vacancy were established, and density functional theory (DFT) calculations were employed to reveal the effects of oxygen vacancy on the surface electronic structure, adsorption configuration and energy of nitrate, reduction pathway, competitive reaction and product selectivity. The results show that oxygen vacancy changes the adsorption configuration and significantly reduces the adsorption energy of nitrate on the surface, and as a result, shifts the potential determining step (PDS) from nitrate adsorption to the subsequent hydrogenation processes. In addition, oxygen vacancy dramatically increases the desorption energy of the intermediates NO2 and NO, and thus inhibits the formation of by-products and improve the electrocatalytic selectivity. Promotion of the competitive hydrogen evolution reaction (HER) by oxygen vacancy is less pronounced than that of nitrate reduction reaction. Therefore, oxygen deficient TiO2-x is a promising catalyst for electro-catalyzing nitrate reduction to produce ammonia.