Abstract: The effects of light intensity,bacteria to algae ratio,and initial ammonia nitrogenconcentration on the removal of ammonia nitrogen were investigated in the Chlorella-bacteria agent sys-tem. The parameters were optimized by response surface analysis. The pathway of NH3-N removal andthe mechanism of algae-bacteria symbiosis mechanism were discussed. The results showed that whenthe light intensity was 4 000 lux,the bacteria-to-algae ratio was 3 ∶ 1 and the initial ammonia concen-tration was 50 mg·L-1,the ammonia removal ( %) reached the highest ( 94.92%) . The factors rankedfrom high influence to low influence on the removal of ammonia nitrogen was light intensity > initial am-monia nitrogen concentration > bacteria to algae ratio. Under the optimal parameters of light intensity of4 700 lux,bacteria algae ratio of 3 ∶ 1 and initial ammonia nitrogen concentration of 50 mg·L-1,themeasured and predicted ammonia nitrogen removal ( %) were 87.96% and 87.31%,respectively. Whenthe ammonia nitrogen concentration was 50 mg · L-1,the microalgae and the bacteria contributed75.86% and 17.50% of ammonia nitrogen removal,respectively. The bacteria maintained the pHbalance of the system and the EPS,as a key medium for the mutual beneficial symbiosis of algae andbacteria,maintained the virtuous cycle of the system. The key bacteria genera which was mutually bene-ficial symbiosis with microalgae could maintain relative stable in the system. Close-