The upper reach of Jinghe Basin is one of the main water-head areas and key afforestation areas in the Loess Plateau of China. A relatively large scaled evaluation of the forest hydrological impacts in this region is important for guiding the rational ecological afforestation, ensuring the regional water supply safety and sustainable development. In order to possibly remove the disturbances from topography, soil and climate, the upper reach of Jinghe Basin was divided into the stony mountain area and the loess area, and several scenarios were set up in each area. The distributed and eco-hydrological watershed model of SWIM was calibrated and validated by using the meteorological and hydrological data measured in 1997-1999 and 2000-2003 respectively, and then was used to simulate the impacts of varying forest cover and its spatial distribution on the annual evapotranspiration, water yield, groundwater recharge, deep soil percolation, and the peak daily runoff. The simulated results in the stony mountain area showed that increasing forest cover will increase the basin evapotranspiration and reduce the water yield. For example, when compared the current forest/vegetation scenario (13.8% of the whole basin) with the scenario of changing all forests into grassland (0% of the whole basin), the basin annual evapotanspiration is changed from 445.4 mm to 427.7 mm (a decrease of 17.4 mm and 4%), the annual water yield is changed from 42.4 mm to 53.5 mm (an increase of 11.1 mm and 26.3%), the annual groundwater recharge is changed from 61.6 mm to 76.9 mm (an increase of 15.3 mm and 24.8%), the deep soil percolation is changed from 72.9 mm to 88.3 mm (an increase of 17.7 mm and 24.3%). In average, an increase of forest cover of 10% will lead to an increase of basin annual evapotranspiration of 12.8 mm, a reduction of annual water yield of 8.0 mm, and a reduction of annual groundwater recharge of 11.1 mm in the stony mountain area. In the relatively dryer loess region with deep soil, the forest cover increase will also increase the basin evapotranspiration and decrease the water yield, but in an obviously smaller variation range compared with the stony mountain area where the annual precipitation is higher and the soil layer is thinner. In average, the basin annual evapotranspiration is increased by 9.0 mm, the annul water yield is decreased by 4.5 mm, and the annual groundwater recharge is reduced by 8.8 mm, when the basin forest cover is increased by 10%. In addition, the hydrological impacts caused by afforestation on more gentle slopes are stronger than those caused by afforestation on steeper slopes. When comparing the monthly distribution pattern of forest hydrological impacts, there is also a clear difference between the stony mountain area and the loess area. For example, the significant increase of evapotranspiration is found in the period from May to July, and the decrease of deep soil percolation is found in the period from May to October in the stony mountain area; while in the loess area it is in the periods of May-October and July-October respectively. In addition, the impact of varying forest cover on the peak daily runoff is not significant in the stony mountain area, probably because of the high infiltration ability of the soil with high stone fragment content; but significant (decrease) in the loess area, probably mainly because of the obviously increased rainwater infiltration into soil after afforestation and then the reduced surface runoff generation.