In previous studies， methods for calculating phreatic water evaporation were based on empirical and semi-empirical formulas established by the law of phreatic subsidence， or were based on the Soil-Plant-Atmosphere Continuum （SPAC） model. However， in the section where the drainage ditch is set to control the phreatic water level， the decline in the phreatic water level is affected by the ditch drainage and phreatic evaporation； therefore， it is necessary to establish a seepage mechanics algorithm under specific hydrogeological conditions. Combined with an observation statistical system of observation data， phreatic water evaporation and irrigation regression were discretized into a step function. The drainage ditch with a rolling dam was generalized into the first type of boundary with a stable water level， and an unsteady seepage model of the phreatic water with a stepped function source sink term under the control of the ditch boundary was established. The analytical solution of the model was derived using Laplace transform， and the reliability of the analytical solution was verified using a numerical solution. Based on the solution of the model， a recursive formula for calculating the evaporation intensity of the phreatic water daily by using the dynamic measured data of the groundwater level was established， and the influence of the ditch boundary was discussed. Taking the Huaibei Plain in the Anhui Province as an example， the application process and steps of the method were demonstrated. The seepage mechanics method for calculating phreatic water evaporation for farmland is different from the existing algorithms based on the submergence law of the phreatic level or SPAC model in principle. The new method can clearly reflect the influence of boundary conditions and requires fewer parameters for calculation， and it is thus more convenient to apply.