The ecosystem can regulate regional temperature and humidity through evapotranspiration. The climate in the hilly and gully area of the Loess Plateau in the Three North Shelter Forest System Project (TNSFSP) region is arid and semi-arid, and the cooling and humidifying function of the forest ecosystem is crucial for improving the living environment in this region. Based on the analysis of forest cover changes in this area, in this study, we simulated using the Penman-Monteith formula and investigated the contribution of evapotranspiration by ecosystems to regional climate regulation. The results show that:(1) The forest area in the study region (mainly grassland, cropland, and desert) increased by 2.25% from 1980 to 2015. (2) The total amount of forest actual evapotranspiration (AET) in the study area from 1980 to 2015 was between 1.19×10¹⁰ t/a and 1.40×10¹⁰ t/a, and average AET ranged from 219 mm to 257 mm. Absolute humidity from June to September in 1980 to 2015 increased by 0.47-0.55 g/m³ per day, and the relative humidity will increase by 2.87%-3.32% per day. (3) Heat absorption of the forest through evapotranspiration was estimated as 29.15×10¹⁵ to 34.26×10¹⁵ kJ/a, evapotranspiration per hectare was 53.72×10⁸-63.13×10⁸ kJ hm^-2 a^-1), and the daily cooling rate by evapotranspiration was 0.92-1.08℃ d^-1. (4) The amount of forest evapotranspiration increased gradually from 1980 to 2010, but it decreased significantly in 2015, which was mainly caused by the decrease in precipitation. Spatially, forests in Shanxi and Shaanxi provinces, with a larger forest area, had better evapotranspiration and humidification effects. Comparison of the different land cover in 1980 to 2015 showed that the actual evapotranspiration of the forest was still significantly higher than other land cover types. In future, the forest area will be appropriately increased within the carrying capacity of water resources, and the role of forest will be even more crucial in regional climate regulation.