Abstract: In order to find the suitable contraction ratio for measuring flume in U-shaped channel with slope greater than 1/200, 7 contraction ratios were designed to carry out the field experiment in irrigation district with 4 types and 6 gradient. The function relation of relative flow rate and relative water depth were deduced based on the dimension analysis method, and the flow formula of the flume was fitted with the field experimental data. The results showed that: when the slope is greater than 1/100, the measuring flume cannot measure the flow; when the U-shaped channel with a slope between 1/100 and 1/200 and the contraction ratio of the flume is controlled at 0.55 to 0.58, the numbers of Froude in front of the flume are less than 0.5, reaching a high measuring accuracy with the average relative error of 2.22% and the maximum relative error of 5%. Under this condition, the maximum backwater height in front of the flume is 16.6 cm, which does not affect the normal operation of the channel. The research results can provide reference for selecting suitable contraction ratio for measuring flume in steep U-shaped channel.