Abstract: The effect of frost degradation on runoff variability in cold regions has become a hot issue under a changing climate. However, most existing researches on the subject have focused on exploring the relationship between permafrost degradation and runoff. In this study, the relationship between frost degradation and runoff and its variability over the last 30 years are investigated. The study uses the frozen depth data from the Manzhouli meteorological station for the period 1974—2006 and the monthly runoff data from four hydrological stations on the Hailaer River basin for the period 1974—2008. The result shows that the winter discharge has increased 13%-20% at the Hailaer, Bahou, and Honghuaerji three hydrologic stations, and no significant change is found at the fourth one (Yakeshi). The discharge ratio of February to November (Q₂/Q₁₁) has increased significantly, while the maximum frozen depth at the Manzhouli meteorological station has decreased about 50cm for the same time period. These results imply that the winter recession curve has shown a slowing trend mainly due to the effect of frost degradation under climate warming. The hydrologic impact of frost degradation would be explainable by the following three aspects. First, the degradation of frozen ground will lead to the increases in aquifer thicknesses, surface water infiltration amounts, and groundwater storages. Subsequently, the groundwater increment will increase the winter discharge. Second, the circulation of groundwater is moving much more slowly than the surface water does. The enlarged groundwater reservoir will lead to a longer river runoff recession. Third, the response of frost degradation to climate warming is a slow process and its effect on runoff variability is also a gradual one.