Abstract: In order to solve the nonlinear problem of runoff concentration caused by excess rainfall intensity,a method deriving time-variant distributed unit hydrograph (TVDUH) based on the SCS's velocity equation was presented in this paper. A reference rainfall intensity is introduced to represent the hydraulic conditions corresponding to k values. A dimensionless factor formed by the ratio of duration excess rainfall intensity to reference rainfall intensity is added in SCS's equation to make up a new velocity equation that can take into account of the influence of excess rainfall intensity on flow velocity. The modified SCS's equation is used to derive the TVDUH of each rainfall duration during a rainfall event. A rainfall-runoff hydrological model is constructed of TVDUH and the runoff generation module of Xin'anjiang model. The rainfall-runoff hydrological model is used in Peihe River basin to calibrate the reference rainfall intensity. Application is performed in Tanjiahe River basin for validation. Results show that the number of events whose coefficient of efficiency being greater than or equal to 0.9 increases from 42% of time-invariant unit hydrograph to 83%. It is concluded that the presented method can contribute to increase the rainfall-runoff simulation capabilities of watershed hydrologic models observably.