<p>The atmospheric precipitable water (APW) in the Bengal region plays an important role in indicating the abundance of water vapor in the monsoon regions of East Asia and India. Based on the ERA5 reanalysis data, the opposite effects of 10–30-day and 30–60-day water vapor income (WVI) on the increasing trend of APW in the Bengal region from 1979 to 2020 are found. There are obvious 10–30-day and 30–60-day oscillations of summer APW in the Bengal region. The WVI, which is represented by the sum of values at 8 phases of unfiltered WVI, exhibits a declining trend in 10–30-day, while the WVI of 30–60-day shows an increasing trend. On the timescales of 10–30-day and 30–60-day, the weakening phase of the WVI shows the largest downward and upward trends among the eight phases, accounting for 32.4% and 321% of each trend, respectively. During the weakening phase of the 10–30-day timescale, a configuration characterized by stronger negative Western North Pacific Monsoon (WNPM) and weaker positive Indian Monsoon (IM) favored the northward wind anomalies over the Indian subcontinent, leading to a negative trend of WVI in the Bengal region and thereby a negative contribution to the increasing of APW. In the weakening phase of the 30–60-day timescale, a configuration featuring weaker negative WNPM and stronger positive IM contributed to northward wind anomalies over the Bay of Bengal, resulting in an increasing trend of WVI and APW in the Bengal region. This study highlights the importance of the phase configuration of both the WNPM and IM in driving the opposing trends observed in 10–30-day and 30–60-day APW variations within the Bengal region.</p>