[Objective] To provide a scientific method for surface runoff calculation and to estimate monthly runoff coefficient in rainy season for the semiarid region of the Western Heilongjiang Province.[Methods] The Alun River basin which flows through the Western Heilongjiang Province was chosen as the study area. Modeling for the vertical soil profile was achieved based on investigating the physical properties of the underlying surface. Digital elevation model(DEM) and river channel network of the study area was generated by GIS-ArcMap. Algorithm on rainfall-runoff was established by kinematic wave equations. Model validation was carried out by numerical simulation for the observed flow. Monthly runoff and runoff coefficient were estimated through analyzing runoff calculation results in the period from May to October in 2012 and 2013.[Results] The model accuracy was within the allowable range of the error criterion(<0.03); Monthly runoff coefficient in July and August were more than 0.5; Runoff in the period from May to October accounted for 34.2% and 34.7% of annual total precipitation in 2012 and 2013, respectively.[Conclusion] The developed numerical model for rainfall-runoff calculation can be applied to the study area and monthly runoff coefficient in the study period.