The effect of bioleaching pretreatment on N2O generation in sludge composting process was firstly investigated in this study. The relationships among physicochemical factors, N2O and NH3 emission and related functional genes were analyzed in 60 days composting of bioleaching dewatering sludge (BDS) and filter press dewatering sludge (FDS), respectively. The results showed the cumulative amounts of NH3 and N2O emission from the BDS composting system were reduced by 83.52% and 54.76% after bioleaching pretreatment, respectively. The lower moisture and pH, and the higher ORP and the concentrations of NH4+-N, NO3--N and NO2--N were observed in BDS during the composting compared to FDS. Furthermore, bioleaching pretreatment improved the relative abundance of hao but reduced amoA, nirK and norB in the BDS during the composting. The low pH level and the reduction of nirK and norB in BDS were the main reasons mitigating NH3 and N2O emissions, respectively.