Covalent organic frameworks (COFs) have emerged as promising atmospheric water harvesters, offering a potential solution to the pressing global issue of water scarcity, which threatens millions of lives worldwide. This study presents a series of 2D COFs, including HCOF-3, HCOF-2, and a newly developed structure named COF-309, designed for optimized water harvesting performance with a high working capacity at low relative humidity. To elucidate their water sorption behavior, we introduce a hydrophilicity index directly linked to intrinsic properties, such as the strength and spatial density of adsorptive sites. This index is mathematically correlated to the step of water adsorption isotherms. Our correlation provides a predictive tool that extends to other microporous COFs and metal–organic frameworks, significantly enhancing the ability to predict their onset positions of water adsorption isotherms based on structural characteristics. This advancement holds the potential to guide the development of more efficient materials for atmospheric water harvesting.