Abstract: The monthly precipitation data from 1724 to 2022 in the Beijing area were used to define seasonality index (SI) and to quantify seasonal characteristics of precipitation． Trends and fluctuations in precipitation seasonality were analyzed, correlation between precipitation seasonality and ocean-atmosphere indices was explored． Data indicate that from 1724 to 2022, SI in Beijing shows a slight increasing trend, with more pronounced fluctuation characteristics, suggesting that precipitation seasonality may be intensifying． Moving t-test method was used to divide SI series into 13 phases of increase and decrease． Before 1834, SI showed very little changes． After 1834, SI alternated between increase and decrease in roughly 60-year cycles． On a seasonal scale, during phases of decreasing SI, precipitation seasonality decreases, leading to a more even distribution of precipitation throughout the year． Rainfalls originally concentrated in the summer tends to shift towards the autumn, and the spring． During phases of increasing SI, precipitation seasonality intensifies, resulting in more concentrated precipitations． 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 these 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 the summer．