Horton's empirical laws of network composition is studied and applied to six catchments in North Shaanxi Province of China, The relationships between Horton parameters are discussed.Based upon the Horson's laws, a geomorphologic instantaneous unit hydrograph (GUH) model is proposed, in which, the waiting time distribution in each path is supposed to be a Gamma distribution. General expressions for the inital state probability anb the transition probability in GUH are derived. All the parameters but one in the GUH are calculated from Horton parameters and stream flow velocity of each flood event. The unknown parameter is decided by optimization.The proposed model is applied to ten rainfall-runoff events in two catchments, the results are encouraging.