Abstract: Based on the monthly precipitation data from 1724 to 2022 in the Beijing area, we define the Seasonality Index (SI) to quantify the seasonal characteristics of precipitation. This study analyzes the trends and fluctuations in precipitation seasonality and explores the correlation between precipitation seasonality and ocean-atmosphere indices. Our results indicate that from 1724 to 2022, the SI index in Beijing shows only a slight increasing trend, with more pronounced fluctuation characteristics, suggesting that precipitation seasonality may be intensifying. Using a moving t-test method, the SI series is divided into 13 phases of increase and decrease. Before 1834, the SI index showed little change; after 1834, the SI index alternated between increase and decrease in roughly 60-year cycles. On a seasonal scale, during the phases of decreasing SI, precipitation seasonality decreases, leading to a more even distribution of precipitation throughout the year. Rainfall that was originally concentrated in the summer tends to shift towards the autumn, followed by spring. During the phases of increasing SI, precipitation seasonality intensifies, resulting in more concentrated precipitation. The percentage of rainfall in spring and autumn decreases, and then concentrates back to the summer. Among the four ocean-atmosphere factors, the Dipole Mode Index (DMI) exhibits a significant negative correlation with SI. The North Atlantic Oscillation (NAO) and the Pacific Decadal Oscillation (PDO) show more stable negative correlations with SI over longer time scales. In contrast, the Oceanic Niño Index (ONI) demonstrates a more stable positive correlation with SI over shorter time scales. Based on the cyclical characteristics, it is inferred that we are likely in an ascending phase of SI. Therefore, it is necessary to pay attention to flood prevention and drainage efforts in Beijing, especially during Summer.