The most widely used way for water measurement in irrigation areas is to use hydraulic structures, whose flow is mainly measured by sluice combined with gate opening. However, the accuracy of water measurement is limited at present, because the calculation formula of flow rate under different flow regimes varies greatly, and the actual value and theoretical value are different in the field application. Based on the analysis of the characteristics of different flow patterns and the variation rules of flow, combined with the measured data of Ganfu plain irrigation area of Jiangxi province, the flow rate calibration model for flat gate based on CAP flow formula is proposed, and applied practically in irrigation areas.The results show that: (1) when the historical measured data of sluice is relatively complete and the flow pattern is relatively comprehensive, the “flow pattern identification method based on the prior of hydraulic jump occurrence” and the “CAP flow formula” should be selected as the flow rate calibration algorithm, which improves the goodness of fit by 0.033 compared with the traditional method (“traditional flow pattern identification method” and “traditional flow formula”); (2) when the historical measured data of sluice cannot be provided completely, in order to improve the accuracy of water measurement as much as possible, the CAP flow formula should be used as the flow rate calibration algorithm, and the average absolute relative error MAPE is used as the evaluation index, which is 16.78%higher than the traditional flow formula．