Located in alluvial plain, the middle and lower Changjiang River suffers from frequent flood disasters. Embankment is the first barrier to protect life and property along the river. The Jingjiang River reach in the middle Changjiang River is exposed to a more serious flood control situation because of large amount of high flood peak and complex river-lake relation. In this research, the flood risk in the flood protection region enclosed by Songzi Levee of upper Jingjiang River reach is assessed. The flood routing process in mainstream Changjiang River and the studied flood protection region in the presence of levee break during a 1000-year event flood is simulated using a 1-D and 2-D coupled mathematical model with generalized river network of Jingjiang River and Dongting Lake water system. Maximum flood depth, inundation duration, and maximum flow velocity are selected as risk elements to analyze the distribution of flood risk. The article is divided into two parts, one of which is presented here to expound the establishment of coupled mathematical model and the process of calibration and verification.