Heihe basin is an important part of the Chaobai river system, which lies on the upstream of Miyun reservoir. Since 1980s, the government has launched many activities on ecological protection and afforestation. In order to understand the impact of these activities on water resource in Heihe basin, this paper analyzes the changes of streamflow by using land use data in the 1980s and 2000 and the precipitation - runoff data from 1959 to 2000. The results show: the interannual variation of precipitation and streamflow in Heihe basin from 1959 to 2000 is large, but has generally shown no obvious trend of rise or decline; and the heihe basin area is dominated by woodland, grasslands and farmland. From 1980s to 2000, the area of woodland had increased by 800 hectares while grassland and farmland had decreased, respectively, by 700 hectares and 100 hectares. During 1980 to 2000, when lots of ecological protection and afforestation activities were carried out, the average runoff coefficient got a reduction of 4.1%, compared to that in the control period of 1959 ~1979. According to the data of two periods in discussion, the impact of ecological protection and afforestation on streamflow is - 0.308mm/hm2 and it indicates a phenomenon that the impact is the largest in wet years, smaller in normal years, and the smallest in dry years.