Abstract: Research was conducted on the thickening mechanism of crude oil during nitrogen injection in fracture-cavity reservoirs to clarify the thickening mechanism and take corresponding countermeasures, thereby enhancing the oil recovery of nitrogen injection. In this study, simulation experiments of nitrogen injection were performed to analyze the influence on crude oil brought by nitrogen extraction, oxygen content in nitrogen, and mixed water. The experiments showed that the oxygen content in nitrogen was the major influential factor. When the oxygen content was 1%, it took only more than 2 days to exhaust the oxygen, and the viscosity reached 18 000 mPa·s, which was 6 times the initial viscosity. When the oxygen content increased to 5%, the viscosity continued to grow to 1 122 000 mPa·s within more than 7 days, which was 366 times the initial viscosity. Emulsification with water and nitrogen extraction resulted in the same effect on viscosity: the viscosity increased 1–3 times. The results demonstrate improving the purity of injected nitrogen is the most effective way to prevent crude oil thickening of fracture-cavity reservoirs during nitrogen injection in Tahe Oilfield. It has provided a theoretical basis for solving the problem of crude oil thickening brought by nitrogen injection in fracture-cavity carbonate reservoirs.