Abstract: To research canal flow-measuring devices is one of the key techniques urgently needed in irrigation districts. Based on Samani. Z and Magallanez. H's researches on semicircular cylinder flume in rectangular canal, three contraction ratios (0.36, 0.49, and 0.57), which were different from theirs, were selected and 48 groups of experiments with these ratios were conducted in rectangular canal. The experimental results showed excellent correlation with Samani. Z and Magallanez. H's 20 groups' and Hager's 12 groups' experiment data, and the coefficient of correlation is R²=0.9958. The fitted exponent-form flux formula is simple, and the average error of the discharge calculation is 3.03%, maximum submergence extent is 0.85.