Based on Preissmann implicit scheme for one-dimensional Saint-Venant Equation,  the mathematical model for one- dimensional river networks and canal networks is developed and the key issues on the model are expatiated particularly in this paper.  This model is applied to simulating the tree-type irrigation canal networks and complex looped canal networks.  The results of levels and flows and discharge distribution between the branches agree with the data from Adlul.  This model is a simple and practical tool for water resource regulation of irrigation canal networks and river networks.  These results show the application value of this model is to set up ecological numerical model of water quality in river networks and canal networks.