This study was investigated agricultural water supply system of major agricultural waterway for Gimje canal, Jeongeup canal, Dongjin river conduit of Dongjin river basin. Furthermore, this result will be used for water resources and agricultural demand in Saemangeum reclaimed arable land. Annual precipitation for 5 years in Dongjin river basin was 1,311.7mm. The average discharges in Dongjin river basin was $1,390{ imes}10^6;m^3$ and $1,516{ imes}10^6;m^3$ and $744{ imes}10^6;m^3$ for 2,007 and 2008, respectively. Also, annual average amount of water resources was 1,861${ imes}10^6;m^3$ and $2,279{ imes}10^6;m^3$ and $1,227{ imes}10^6;m^3$ for 2,007 and 2008, respectively. Dongjin river basin water system for the analysis of agricultural water in water resources, runoff, agricultural water demand and usage surveys were analyzed, resulting in the total amount of water due to precipitation of the watershed of the $12.3{ imes}10^9;m^3$ ~$22.8{ imes}10^9;m^3$ and Dongjin River basin in waters flowing discharge is $7.4{ imes}10^9;m^3$~$16.1{ imes}10^9;m^3$, agricultural water demand and usage of each of $6.8{ imes}10^9;m^3$~$6.9{ imes}10^9;m^3$ and $4.9{ imes}10^9;m^3$~$7.1{ imes}10^9;m^3$ compared to the agricultural water demand was more likely. Agricultural water supply system in Dongjin river basin is complex because of devided branches to the main canal and branch canal. In this process, accurately assessment of water usage is very difficult. Therefore, systematic management of water resources and supply of agricultural water supply system to use the terms of the complexity and diversity by considering the appropriate level of agricultural water management systems will be needed. As a result of this study, it can be used water resources assessment in quantity, rational usefulness and basic planning of water resources development for water distribution.