In hydrological experiments, the flux (m3) of various types of weir was well studied while the study on the accuracy of the calculation of the amount of water discharge was neglected. A new integral equation method was developed and applied to improve the accuracy of the calculation of the amount of water discharge. The water level hydrograph of each rainfall-runoff event was divided into periods at ten minute intervals. In each period, a water level- time linear function was established. Then, the water level-flux relationship was transferred into a function between flux and time in each period for integration. Thus a new integral equation method was developed to calculate the amount of water discharge in sub-catchments and catchments. At the same time, the instant flux coefficient method was applied to calculate not only water discharge in each period of ten minute interval, but also day, monthly and annual water discharge. The result showed that the new integral equation method can improve the accuracy of annual water discharge by 8.7%--17. 1% at five monitoring sites using the field observation data in the year of 2000. The integral equation method improved the area-flux method with a mathematical integral equation. Compared with the instant flux coefficient method, the new method improve accuracy greatly and was suitable for the calculation of the amount of water discharge at weirs on various conditions of water level variation.