The number of reservoirs in the world is increasing year by year, which will inevitably affect the downstream runoff and baseflow recession characteristics. In this paper, the impact of Chaersen Reservoir (Northeast China) on downstream runoff and baseflow recession characteristics is studied using a pre post comparison method, and the influence of climate change is excluded using two upstream sub-watersheds as control basins. In addition, the impact mechanism of the reservoir is further explored. The results show that the increased direct and indirect water consumption after the construction of the reservoir results in a 14% reduction in the average streamflow of the downstream Zhenxi Station. At the same time, the construction of the reservoir causes the baseflow at the recession stage to increase by a relatively constant value (about 1m3/s), which leads the log(|dQ/dt|) vs. log(Q) points at the low flow stage to shift to the right. This eventually results in a decrease in the recession coefficient a by about 60% and an increase of b by about 24%. The log(|dQ/dt|) vs. log(Q) scatter of the recession process after adding a constant flow is no longer in a strict linear relationship. And the master recession curve obtained by the traditional linear parameterization method is only an average approximation, which will overestimate the streamflow in the middle recession stage and underestimate the streamflow in the late stage.