Both the time series analysis method and hydrological modeling approach are integrated to analyze the streamflow response to climate variability and human activities in the Bai River basin, northern China using data from 1986 to 1998 in this study. Also, the quantification and separation of effects from climate variability and human activities is investigated. First, the Fu formula based on Budyko hypothesis was applied to explore the integrated underlying surface characteristics in the whole basin, and then the SIMHYD model was calibrated and validated using the data from 1986 to 1990 (pre-treatment period). The calibrated model was then used to simulate streamflow in the period 1991 to 1998 (testing period) and obtain quantitative assessment on the impacts of climate variability and human activities. The difference of observed streamflows between the pre-treatment period and the testing period reflects the combined influence of climate variability and human activities in the basin, while the difference between simulated and observed streamflow during the testing period reflects the impact of human activities in the catchment. The results show that the contribution rate of climate to the streamflow change in the basin is 37.5 and 62.5% for human activities. Human activities exerted a dominant influence upon streamflow change in the Bai River basin.