Abstract: The development of sensor technology has brought new opportunities to the field of channel flow measurement. At present,most flow measurement equipment mainly estimates the flow of the channel section by measuring the flow velocity at the point. However,in the channel,due to the effect of backwater,the velocity distribution at the same section changes with the downstream water depth. In order to explore the cross-section flow velocity distribution characteristics of open channel under different water depth boundary conditions,this paper uses FLOW-3D for 3D modeling and simulation. A 100 m canal section in a typical trapezoidal channel in the plain irrigation area is selected to perform constant flow for different downstream backwater conditions. By studying the cross-section velocity distribution under different downstream water depth conditions,the possible flow error caused by using a fixed surface velocity coefficient is analyzed,and according to the variation law of the water surface velocity coefficient,a fixed-rate method is proposed when the height of the backwater below the channel is less than 20 cm water surface velocity coefficient can be used to estimate the flow error（the error is less than ± 10%）. This paper also fits the simulation data to obtain the curve of the water surface velocity coefficient with respect to the water depth,and uses this curve to modify the water surface velocity coefficient. The results show that the measurement error can be reduced to within 1.4%. Finally,it analyzes the minimum number of measurement points that should be selected when the radar technology is used to measure the water surface velocity,while ensuring that the measurement error is no more than 10%.