Water electrolysis derived by renewable energy such as solar energy and wind energy is a sustainable method for hydrogen production due to high purity, simple and green process. One of the challenges is to reduce energy consumption of water electrolysis for large-scale application in future. Cell voltage, an important criterion of energy consumption, consists of theoretical decomposition voltage (Uθ), ohmic voltage drop (i⁎∑R) and reaction overpotential (η). The kinetic and thermodynamic roots of high cell voltage are analyzed systemically in this review. During water electrolysis, bubble coverage on electrode surface and bubble dispersion in electrolyte, namely bubble effect, result in high ohmic voltage drop and large reaction overpotential. Bubble effect is one of the most key factors for high energy consumption. Based on the theoretical analysis, we summarize and divide recent intensification technologies of water electrolysis into three categories: external field, new electrolyte composition and new thermodynamic reaction system. The fundamentals and development of these intensification technologies are discussed and reviewed. Reaction overpotential and ohmic voltage drop are improved kinetically by external field or new electrolyte composition. The thermodynamic decomposition voltage of water is also reduced by new reaction systems such as solid oxide electrolysis cell (SOEC) and carbon assisted water electrolysis (CAWE).