In this paper after simulating a dam with GeoStudio software, the place of phreatic curve, intersection it with the downstream slope, the drop before the core and also passing through it, were analyzed and the results were compared with Rezk's experimental solution. Also, the effect of changes in the water level of the reservoir, the relative value of the different permeability and changes of the slope of  the upstream and downstream on the phreatic curve have been analyzed. The results of this research show that phreatic line with the relative value of the same permeability is not dependent on the value of the permeability. Also, the relative value has been assumed (k_c/k_d) = 0.1, in Rezk's experimental solution, which has a 25% error. We have reduced the error to less than 3% with the extension method of Rezk in the lower Permeability ratios. In this case, Rezk's experimental solution and the results of finite element method show excellent agreement. Also it was determined that the error rate changed by changing the geometry of the dam so that the error decreased from 43% to 25% by reducing the slope of the upstream.