Vapor pressure deficit (VPD), which defined as the difference between the saturated vapor pressure (SVP) and the actual water vapor pressure (AVP) at a selected temperature, is a key variable for measuring atmospheric aridity. It also plays an important role in modulating vegetation photosynthesis, carbon, and water exchanges. The continuously increasing VPD will lead to forest mortality, crop failures and reduced plant biomass. Analyzing its spatiotemporal characteristics and influencing factors provides a deeper understanding of the response mechanisms of terrestrial ecosystems to climate change. This study uses 126 site observations to evaluate the performance of the monthly scale VPD calculated by the Climate Research Unit gridded Time Series (CRU TS) 4.07, the fifth-generation European Centre for Medium-Range Weather Forecasts Atmospheric reanalysis version 5 (ERA5), and Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA2) at the Hai River Basin in the period of 1980-2022. Annual and seasonal VPD change trends and influencing factors were also evaluated using the Sen's slope estimator and correlation analysis in the Hai River Basin over this period. The results show that: (1) Monthly VPD estimated from the ERA5 dataset has smaller BIAS and root mean square errors (RMSE) compared to the other two grid datasets, with R²=0.9851. Therefore, the ERA5 dataset is the most suitable dataset for studying long-term VPD changes in the Hai River Basin. (2) There was a significant increasing trend of annual VPD in the Hai River Basin from 1980 to 2022, with a rate of 0.027kPa/10a (P < 0.01). VPD exhibited an upward trend across all four seasons, with the highest rates in spring (Trend=0.0433kPa/10a, P < 0.01) and summer (Trend=0.0405kPa/10a, P < 0.01) and the lowest in winter (Trend=0.010kPa/10a, P < 0.01) and autumn (Trend=0.0146kPa/10a, P=0.051). (3) Significant increases in annual VPD were observed in 86.28% of the Hai River Basin (P < 0.05), and the strong increase were mainly located in the southeast of the basin. Seasonal increases in VPD were significant in spring (87.59%), summer (63.57%), autumn (30.76%), and winter (77.48%). (4) The annual and seasonal VPD in the Hai River Basin showed significant positively correlations with SVP and significant negative correlations with AVP, with a stronger correlation between VPD and SVP than AVP. Additionally, the absolute values of the correlations between VPD and relative humidity were higher than those between VPD and temperature, indicating that relative humidity has a greater impact on VPD than temperature. Overall, our findings emphasize the importance of using high-quality climate datasets to assess the spatiotemporal characteristics and influencing factors of VPD.