Nitrogen transport in runoff has a significant effect on nitrogen loss. In order to observe the effect of Polyacrylamide (PAM) addition on ammonium nitrogen transport in runoff, experiments were conducted with rainfall simulator under laboratory conditions. The results showed that PAM addition decreased ammonium nitrogen concentration dissolved in runoff, which was significant in initial reduction of ammonium nitrogen concentration, and it turned to a stead process of ammonium nitrogen loss. Based on the mixing zone theory and nitrogen mass conservation principle, a non-uniform and incompletely model was established to describe ammonium nitrogen transport in runoff. The model analytical result indicated that ammonium nitrogen concentration in runoff could be expressed as an exponential decrease with time, and was in accordance with the experimental data.