The effect of Ipomoea aquatica floating-bed on the quantity and distribution of nitrogen cycling bacteria and nitrogen removal was investigated in plastic tanks using I. aquatica as floating-bed plant. The results showed that the number of total nitrogen cycling bacteria, ammonifying bacteria, nitrosobacteria, nitrobacteria in the I. aquatica floating-bed system was significantly higher than that in the control (P<0.01). No significant difference was found for the number of denitrifying bacteria between I. aquatica floating-bed system and the control (P>0.05). The number of total nitrogen cycling bacteria in I. aquatica floating-bed (7×10⁵-3×10⁸ MPN/g) was 2 to 5 orders of magnitude higher than that in the control (3×10²-5×10⁶MPN/g). The dominant group in the I. aquatica floating-bed system was ammonifying bacteria throughout the test period (P<0.01). But the dominant group in the control had changed during the test period. The dominant bacteria group in the control was ammonifying bacteria at the sixth and twelfth day of sampling (P<0.05), while ammonifying bacteria and denitrifying bacteria were dominant at the nineteenth and thirty-third day of sampling (P<0.05). The dominant group in the control was denitrifying bacteria at the twenty-sixth day of sampling (P<0.05). The difference in the distribution of ammonifying bacteria was not significant among endo-rhizosphere, rhizoplane and water. Nitrosobacteria was mainly distributed in the endo-rhizosphere and rhizoplane. Nitrobacteria was mainly distributed in the rhizoplane and denitrifying bacteria was mainly distributed in the water. The removal rate of NH₄⁺-N was 91.8% in the I. aquatica floating-bed system and 88.5% in the control, respectively. NH₃ volatilization could be the main way of NH₄⁺-N removal in the control. Microbial nitrification could be the main way of NH₄⁺-N removal in the I. aquatica floating-bed system. Since I. aquatica covered only 10% of the water surface in the floating-bed system, NH₄⁺-N removal rates in the two systems were not significantly different. The removal rate of total nitrogen was 48.2% in the I. aquatica floating-bed system and 62.1% in the control, respectively. Due to the decay of I. aquatica induced by insects after the 19^th day, the removal rate of total nitrogen in the I. aquatica floating-bed system was lower than that in the control. But the interference of insects offered a possibility to have a thorough knowledge of purification mechanism of nitrogen with ecological floating-beds. Based on the correlation analysis between the number and distribution of the nitrogen cycling bacteria and the nitrogen concentration, we found that the contribution of ammonifying bacteria and nitrosobacteria in the rhizoplane is higher than that in the endo-rhizosphere and water. The contribution of nitrobacteria and denitrifying bacteria in the water is higher than that in the endo-rhizosphere and rhizoplane. Nitrogen removal is the combined effect of plant absorption, ammoniation in the rhizoplane, nitrification and denitrification in the water in the I. aquatica floating-bed system. But the main way of nitrogen removal is microbial nitrification, denitrification and NH₃ volatilization in the control. The results provide sound basis for further understanding about the purification mechanism of nitrogen with ecological floating-beds.