The intra-annual distribution of precipitation has a significant impact on vegetation growth in arid and semi-arid regions. Intra-annual variations in tree-ring cellulose oxygen isotope ratios (S18Otree) can capture seasonal climate signals. In this study, we collected tree-ring cores of Picea crassifolia from three sampling sites in the northeastern Tibetan Plateau and established both interannual and intra-annual S18Otree series spanning approximately the past 30 years. We found that all three sites exhibited a consistent pattern of intra-annual variation, with S18Otree values gradually decreasing from earlywood to latewood reflecting the relative humidity of the corresponding growth periods. Further analysis revealed that the amplitude of intra-annual variations in S18Otree can indicate the intra-annual distribution of precipitation, specifically the difference in precipitation amounts between the late and early growing season. Additionally, when examining the relationship between annual-resolution and intra-annual-resolution S18Otree series, we found that annual-resolution S18Otree primarily reflect the isotopic signals corresponding to the periods of fastest tree growth within the year. Our findings provide valuable insights into the interpretation of annual-resolution S18Otree signals and the investigation of seasonal moisture variations in arid and semi-arid regions under the context of climate change.