The curve of water fractional flow is of vital importance to the research on reservoir development，which can be achieved by laboratory test. The endpoint value（water saturation）of this curve cannot be obtained directly owing to insufficient injection volume limited by experimental period and instruments. Also，the endpoint value cannot be calculated by fitting because current calculation models neglect limit displacement. Based on the reservoir engineering theory，the theoretical relationship，characteristics，and boundary condition of the curve of water fractional flow were specified. The rational formula is chosen for building the whole process model of water fractional flow through the comparison of multiple mathematical models. The results show that the rational formula model has high accuracy as the average relative error between calculation value and test value is 0.4%，for which it can be used for forecasting the residual oil saturation data at the point of limit displacement and calculating the whole process curve of relative permeability ratio. This rational formula model has great practicality and can be widely adopted because the parameters in the calculation are accessible in the test report on relative permeability. More importantly，it can provide some guidance for the theoretical limit，surplus potential，and direction adjustment of the development of water drive reservoirs.