The invention discloses a flood analyzing method based on a one-dimensional unsteady flow numerical model. According to the flood analyzing method, when the one-dimensional unsteady flow numerical model of a river region to be analyzed is set up, a river network model is generalized into two parallel river channels including a main channel and an approach channel, a longitudinal construction cofferdam between the two parallel river channels is generalized into a connecting channel connected with the two parallel river channels, then flood levels nearby all influence objects in the river region are obtained in the one-dimensional unsteady flow numerical model according to inflow conditions, and therefore the influence on the water level below a dam by construction cofferdam breach during ship lock construction stage in the river region can be analyzed.