According to the water balance principle, the expression of the phreatic table is established by considering evaporation and considering evaporation & drainage respectively. The law of phreatic table variation under the combined effects of considering evaporation & drainage is obtained. Based on Newton-Raphson itera-tive method, calculation and analysis programs are done to solve the first order implicit differential equation of the sewer drainage calculation considering phreatic evaporation. The calculation results show: after considering phreatic evaporation, in the need of reducing the same height of phreatic, the sewer spacing is appropriately larger than the case which didn’t consider the affection of phreatic evaporation; the phreatic drop speed in both effects of evaporation and drainage is faster than the case of just considering the drainage effect. Phreatic evaporation has a great influence to the subgrade sewer distance, phreatic evaporation needs to be considered in the subgrade drainage design. According to the water balance principle, the expression of the phreatic table is established by considering evaporation and considering evaporation & drainage respectively. The law of phreatic table variation under the combined effects of participating evaporation & drainage is obtained. Based on Newton-Raphson itera- tive method, calculation and analysis programs are done to solve the first order implicit differential equation of the sewer drainage calculation considered phreatic evaporation. The calculation results show: after considering phreatic evaporation, in the need of reducing the same height of phreatic, the sewer spacing is decent larger than the case which did not consider the affection of phreatic evaporation; the phreatic drop speed in both effects of evaporation and drainage is faster than the case of just considering the drainage effect. Phreatic evaporation has a great influence to the subgrade sewer distance, phreatic evaporation needs to be considered in the sub grade drainage design.