Abstract: Dew serves as a vital supplement to regional water resources and plays a significant role in the ecological and hydrological processes, particularly in arid regions and during drought periods. However, existing research on dew is limited, with a primary focus on localized areas and short-term time series. Therefore, this study utilizes dew frequency observations from 628 stations across China and nationwide reconstructed dew frequency data derived from a Random Forest model to analyze the spatiotemporal evolution of dew frequency across China and within different climatic zones. Results indicate: 1) Dew frequency exhibits a significant decreasing trend. The decreasing rate at the station scale is -0.54 days per year, while the decreasing rate at the national scale is -0.23 days per year, with decreasing areas covering 35% of total national land area. 2) Dew frequency demonstrates pronounced seasonal variations within the year, manifested as uneven monthly distribution and divergent temporal trends. Specifically, dew occurs predominantly from May to October, peaking in August, while months with significant decrease in dew frequency cluster from April to September. 3) Dew point temperature, representing air humidity conditions, is the key factor influencing dew formation. While topographic dynamic conditions such as wind speed and elevation exert a certain regulatory effect on dew formation. 4) Dew frequency decreases across all climate zones, with water-stressed arid zones being the most sensitive. The arid zone has an average dew frequency of 66 days, with areas experiencing significant decrease covering 41.70% of its total area and the decreasing rate of -0.16 days per year. Consequently, the availability of non-precipitation water sources in arid zones is further decreasing, regional water stress is increasing, and ecosystem risks are intensifying simultaneously, posing severe challenges to water resource regulation and management.