With the exploration and development of deepwater/ultra-deepwater oil and gas resources, drilling fluids must withstand extreme temperature differences ranging from seabed low temperatures (0–4 °C) to downhole high temperatures (>200 °C), posing severe challenges to their rheological properties. To overcome issues such as low-temperature thickening and high-temperature viscosity reduction in conventional water-based drilling fluids across wide temperature ranges, developing intelligent rheological modifiers with “flat-rheological” characteristics has become a key technological direction. This paper systematically reviews recent advances in relevant research both domestically and internationally, focusing on four types of rheological modifiers: linear polymers, nonlinear polymers, biopolymers, and nanocomposites. It comprehensively summarizes the constant rheological mechanisms and performance boundaries of these modifiers, identifies the main bottlenecks in current technology, and outlines the future development pathways.