Subsurface drainage system is an efficient system to drain extra flood out from the farmland. But this system also changed the transport pathway and speed of water and the dissolved pollution in farmland. To get a full understanding of vertical transportation of nitrate and phosphate in subsurface drainage agricultural watershed, finite element analysis technology was used to make the numerical simulation of mass transfer of soil column. Results show that there is no water and dissolved pollutant in pipe after 5 h rainfall with intensity of 4 mm/h, because the rainfall in 5 h is not enough to saturate the soil layer from surface to pipe. But if extending the rainfall duration to 10 h, the soil became saturated and then the nitrate and phosphate loss observed. The nitrate loss was extremely higher than phosphate ( nitrate concentration is 9-10 mg/L,phosphate is 0.05-0.1 mg/L) . After 24 h rainfall event, the nitrate loss reached to the maximum. The nitrate loss in tile water intensity was at an exponential growth but phosphate just increased at a liner growth. This phenomenon may be caused by the differences of material properties.%浅层排水系统是一种快速排出农田积水的有效设施，但其同时也改变了水分及面源污染物如氮磷在农田系统迁移的速度和途径．为了对浅层排水农田系统中的氮磷垂直迁移过程有全面了解，利用有限元分析技术对已监测的浅层排水系统单元土壤柱进行数值仿真．结果表明，在5h4mm／h降雨结束后，降雨量没有达到使地表到排水管道之间土壤层完全饱和，从而使随之溶解的氮磷污染物不足以渗透到排水管，导致没有氮磷的流失．降雨时间延长至10 h后，降雨量使土壤层达到饱和，从而使溶解的污染物产生流失，其中硝酸盐的流失量明显大于磷酸盐，硝酸盐流失量为9～10 mg／L，磷酸盐流失量为0．05～0．1 mg／L．在24 h降雨事件后，排水管中硝酸盐质量分数达最大值．在降雨时间不断增加的过程中，硝酸盐流失呈指数增长，而磷酸盐只是线性增长，形成该现象的原因与污染物的本身性质有关．
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