Nitrogen is an important limiting growth factor in alpine and polar ecosystems. At present, the research on the mechanism of plant N utilization in permafrost area mainly focuses on the complete thawing period, while the strategy of plant N absorption in the freeze-thaw cycle is still uncertain. In this study, the absorption and preference of plants for (¹⁵NH₄)₂SO₄ and Na¹⁵NO₃ were analyzed by isotope tracer technology in freezing period (late autumn) and thawing period (early spring). The results showed that the growth of Stipa purpurea canopy had stagnated in the freezing period, and its canopy had not turned green in the thawing period, but the root system and standing dry could absorb and assimilate ¹⁵N, which had the need to absorb nutrients. The total recoveries of ¹⁵N-NH₄⁺ and ¹⁵N-NO₃^- in thawing stage were 4.44% and 6.91% respectively after 21 days of ¹⁵N labeling, while the total recoveries of ¹⁵N-NH₄⁺ and ¹⁵N-NO₃^- in freezing stage were 8.65% and 3.55%, respectively. In the early stage of thawing, the overall recovery of ¹⁵N-NH₄⁺ was higher than ¹⁵N-NO₃^-, but in the later thawing stage of thawing, the recovery of ¹⁵N-NH₄⁺ and ¹⁵N-NO₃^- was not significant or ¹⁵N-NO₃^- was slightly higher. The recovery rate of ¹⁵N-NH₄⁺ was higher than that of ¹⁵N-NO₃^- in the whole freezing period. This study supplements the nutrient absorption data of plants in the non-growing season in the alpine permafrost region, and provides a scientific basis for a comprehensive understanding of plant N nutrition strategy in the permafrost region and its response to future climate change.