Abstract: In order to improve the flow measurement accuracy of the new ultrasonic square measuring flume under the condition of non-submerged outflow,hydraulic contraction sections were added before and after the square flume and hydraulic formula was introduced to assist the flow measurement. The flow error comparison test was carried out with the triangular weir. Results show that the flow measurement error of square channel without contraction section is less than 8% under the condition of submerged outflow; the flow measurement error of square channel with contraction section is less than 8%when the flow rate is greater than 0.1 m3/s,and the calculation error of flow formula in the contraction section is less than 5% when the flow rate is less than 0.1 m3/s. The method of flow measurement assisted by contraction section and hydraulic formula can improve the accuracy of flow measurement of square channel under the condition of non-submerged outflow,furthermore,the applicability of the square channel is improved. It lays a foundation for the application of ultrasonic square measuring flume in actual water measurement. This new type of flume with high precision,strong adaptability and simple flow measurement can save the manpower and material resources needed for open channel water measurement,and promote the construction process of information water measurement in irrigated areas.