Based on data from 53 rain gauge stations, ten runoff stations, and four meteorological stations in the Laohahe Basin, a typical semi-arid area in northern China, we studied the temporal and spatial variations of key hydrological factors in ten sub-basins of the Laohahe Basin over the past 50 years. We employed the rank-based Mann-Kendall trend test and Pettitt change-point test to analyze the change trends and significance levels of precipitation and runoff in the basin and calculate the change point. Then, we adopted a semi-distributed variable infiltration capacity (VIC) model to simulate the rainfall-runoff processes of the ten sub-basins during the baseline period. Based on calibrated parameters of the model, we reconstructed natural runoff during the variation period, so as to quantitatively separate the effects of climate change and human activities on the abrupt reduction of the runoff in the basin. The results show that since 1980, 90% of the runoff reduction in the basin has been caused by human activities, and 10% of the runoff reduction has been caused by climate change. In terms of human activities, widespread agricultural irrigation especially in the middle and lower reaches, was the essential reason for the abrupt reduction of runoff in the Laohahe Basin. Furthermore, the impact of human activities on runoff reduction varied with dryness and wetness changes across the basin, with greater impact in dry years than in wet years.