Nitrogen loss through gaseous emission, mainly ammonia emission, was an inevitable problem during sewage sludge composting. In this study, MgSO4 + K3PO4 (Run A), K2SO4 + KH2PO4-K2HPO4 (Run B) and MgSO4 + KH2PO4-K2HPO4 (Run C) were mixed with mixtures before composting, aiming at researching the effects of buffer solution on reducing nitrogen loss during composting. Ammonia loss of Run C was reduced by 53.8% and 45.5%, and nitrogen loss of Run C was decreased by 61.2% and 67.1%, compared to that of Run A and Run B, respectively. Besides, organic matter degradation of Run C was 36.8%. Among the three amended treatments, nitrogen loss in Run C was effectively reduced and organic matter degradation was slightly improved. The addition of MgSO4 and KH2PO4-K2HPO4 was confirmed to be effective to maintain a desired pH range for struvite precipitation as well as to reserve more ammonia in the compost to promote the formation of struvite.