In this paper, a self-similar river networks model was put forward, which uses string codes to identify the original unit and the generator unit of RSN (Random Self-similar River Network). These string codes are consist of four parameters: the river networks price, the node number in generator unit, the angle between adjacent chains, and the probability of replacement of generator unit. Set the basic part of limit distribution of water system as the generator unit, and then use the Width Function (WF) of RSN to construct a Geomorphologic In - stantaneous Unit Hydrograph (GIUH) based on WF, which is used to simulate hydrological response.