A gel filler with diatomaceous earth, activated carbon, and CaCO3 compounded with polyvinyl alcohol was prepared and optimized. Rapid start-up and efficient and stable operation of partial nitrification (PN) was achieved by immobilization of conventional activated sludge. The results showed that the proper material proportion synergistically improved the performance and oxygen mass transfer rate of the gel. The continuous flow reactor was operated for 10d to successfully start PN, and the maximum ammonia oxidation rate of this filler was 58.30 mg·(L·h)−1 during the stable operation, and the nitrite accumulation rate was maintained above 90 %. However, due to the greater mass transfer resistance of unoptimized fillers, more dissolved oxygen waste and nitrogen loss were caused, resulting in delayed expression of biological activity. Based on the kinetic characterization of ammonia-oxidizing bacteria (AOB) within the filler and high-throughput sequencing, it was demonstrated that the rapid start-up and efficient and stable operation of the PN system was due to the highly oxygenophilic and community-dominant position of AOB within this filler. This study provides new options and insights at the level of immobilization technology for the realization and efficient and stable operation of PN.