Recently, mainstream anaerobic ammonium oxidation (anammox) has attracted extensive attention due to its energy-neutral advantages. The inhibition of nitrite oxidizing bacteria is a key factor to guarantee the occurrence of the anammox process, which is usually achieved by controlling the aeration conditions. This study integrates the gel entrapment technique with a traditional biofilm (abbreviated as BSgel system) to realize a simultaneous partial nitrification, anammox and denitrification (SNAD) process. The anammox biofilm on the surface of the carrier was covered by the gel with entrapped activated suspended sludge. The results show that the SNAD process successfully took place in the BSgel, while the traditional hybrid system of biofilm and suspended sludge only yielded the nitrification process. Furthermore, the hindrance effect of high organic matter concentration (up to 100 mg L⁻¹ COD) was weakened and the specific total inorganic nitrogen (TIN) removal rate reached 57 mg N (gVSS d)⁻¹ in the BSgel. However, the low temperature (15 °C) reduced the nitrogen removal potential. Besides, the BSgel system showed a reactivation ratio of nitrogen removal ability of 68% after preservation at −20 °C for one month. The high-throughput sequencing analysis indicates that the BSgel system combined the common microbial community characteristics of both the biofilm and suspended sludge. The dominant anammox group belonged to Candidatus Kuenenia. Finally, the numerical modelling predicts the microbial and nutrient profiles in the gel film of partially entrapping and fully entrapping BSgel systems, indicating that the fully entrapping pattern outperformed the partially entrapping pattern. Overall, our findings support that the BSgel system facilitates the SNAD process under mainstream conditions within a short time.