Abstract: Microorganism immobilization technology is used in situ to purify polluted water. To explore effect of the technique purifying polluted ponds and mechanism of the microorganisms involved in N removing denitrification in the purifying process, and to elucidate relationships between microbial abundance and water properties, spatiotemporal variation of the abundances of 16S rRNA and denitrifying genes (nirK, nirS, and nosZ) in water was analyzed using the realtime PCR technique, while changes in water quality (COD_Mn, NH₄⁺, NO₃^-, NO₂^-, TN, TP) of the ponds that accept domestic sewage were monitored from March to May, under subtropical climate. Results show that compared with the influent water, the water in the treated pond was much lower in concentration of COD_Mn, NH₄⁺ and TN and much higher in transparency. Total bacteria and nosZ-type denitrifying bacteria varied in a similar pattern, peaking on D3 in abundance (3 days after the treatment was done) up to 2.58×10⁸ and 2.98×10⁴ copies·mL^-1， respectively, on average, while nirK- and nirS- types of denitrifying bacteria did not varied much in abundance in the initial period of the experiment, and then soared up after D30. The parameters of water quality had much influence on bacterial abundance. The abundance of 16S rRNA genes of total bacteria, which was closely related to pH and transparency (R=0.431 2 and -0.659 7， P<0.001) of the water, while, the abundance of nirS type denitrifying bacteria was to temperature (R=0.789 9, P<0.001), and concentration of NO₃^--N, NO₂^--N and TP （R=-0.555 9, -0.756 9 and -0.446 3，P<0.001) and the abundance of nosZ-type denitrifying bacteria was to transparency (R=-0.453 4，P<0.001), and concentration of NH₄⁺-N and PO₄^3--P(R =-0.527 2 and -0.491 4，P<0.001). All the above findings indicate that the microorganism immobilization technique is applicable to in-situ purification of polluted waterbodies through growth of the highly efficient denitrifying bacteria attached on the film and harmonization of environmental factors for growth of the native bacteria in the water, especially denitrifying ones. The interactions of the two effects make it possible to have polluted water purified.