Water-based drilling fluid is faced with complex conditions such as high temperature and high salinity in the formation, and cuttings intrusion, which can cause severe thickening of the drilling fluid, resulting in excessive pump start-up pressure, pipe sticking, wellbore instability, and other issues. Viscosity reducers are crucial for regulating the rheological properties of drilling fluid and preventing the thickening. The mature viscosity reduction theory is the basis of developing high-performance viscosity reducers. In this paper, the research status of various viscosity reduction theories (drilling fluid network structure destruction theory, dispersion inhibition of clay hydration theory, and hydrophilicity reduced theory) as well as different types of viscosity reducers (modified natural materials, synthetic polymers, viscosity reducers prepared from industrial and domestic waste) are reviewed. The shortcomings of existing viscosity reduction theories and viscosity reducers are summarized, the difficulties of theoretical research and material development are analyzed. It is proposed that molecular simulation should be employed to visually validate viscosity reduction theory at the molecular level to guide the selection of efficient functional monomers. Moreover, optimizing the synthesis process, reducing the modification cost, with a focus on the development of low-cost, high-performance, eco-friendly, and intelligent viscosity reducers, is the developing trend in the future.