The expansion of urban impervious surface has a significant effect on surface runoff. Taking the six central regions along the Songhua River in Harbin as the study area,the paper used linear spectral decomposition technique and long-term hydrologic impact assessment (L-THIA ) model to simulate four daily rainfall scenarios such as ligh rain,moderate rain,heavy rain and rainstorm.It simulated the impact of impervious surface expansion on daily and annual runoff variation.Result indicated that the daily runoff in light rainfall scenario is particularly affected by the impervious surface,with the increase of rainfall,the impact is reduced significantly;in term of annual runoff,the impervious surface expansion had more significant impact on dry years than that on wet years.The paper used L-THIA model to simulate the runoff change caused by impervious surface expansion.The method requires a small amount of parameters.The operation is simple and quick.The result can provide a basis for flood mitigation,planning and management of urban．