Supercritical CO, as an environmentally friendly and pollution-free fluid, has been applied in various EOR techniques such as CO flooding. However, the low viscosity of the gas leads to issues such as early breakthrough, viscous fingering, and gravity override in practical applications. Although effective mobility-control methods, such as CO WAG (water alternating gas)-, CO foam-, and gel-based methods, have been developed to mitigate these phenomena, they do not fundamentally solve the problem of the high gas-oil mobility ratio, which leads to reduced gas sweep efficiency. Adding CO direct thickeners to displacing fluid can increase its viscosity, achieve deeper mobility control, and thus improve the CO flooding oil-recovery effect. Unlike other methods, direct thickeners can alter the physical and chemical properties of CO, making it a fundamentally effective means of achieving mobility control. This approach can be applied in various reservoir environments and formations, or it can assist other methods for more in-depth mobility control. This article reviews the development and application of CO direct thickeners and introduces the thickening mechanisms and effects of different types of thickeners as well as their existing problems and future development directions.