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Based on indoor and field experiments, velocity distribution properties of U-shaped open channel were analyzed. Exponent formula for horizontal velocity distribution properties and two-power law for vertical velocity distribution properties were established. The methods to define the parameters were given. Then the midline three-point for flow measurement of open channel was put forward based on the velocity distribution properties of U-shaped channel. The fractional errors of flow velocities calculated by the midline three-point method in indoor experiment under different operating conditions and field experiment were within ±4% and ±5%, respectively. Validations showed that the midline three-point method increased the computational accuracy of flow measurement in open channel. In addition, the workload of the flow measurement by the midline three-point method was substantially reduced by more than 60% compared with velocity-section method which needed nine measuring points.