Improving the recovery efficiency of flue gas waste heat and reducing the emission of nitrogen oxides have always been two issues that need to be paid attention to in gas boilers. Relevant research work has important engineering application value. A spay type flue gas condensation waste heat recovery system integrating combustion air humidification was proposed. The effects of changes in combustion air moisture content on the flue gas dew point temperature, system heat recovery efficiency, and nitrogen oxide emissions of gas boilers were investigated experimentally. The experimental results show that, when combustion air moisture content increases from 3 g/kg to 60 g/kg, the dew point temperature of flue gas increases by 8.0 oC. Under the spray flow rate of 0.075 kg/s and the spray water temperature of 45 oC, the heat recovery efficiency of the system can be increased up to 8.4%. The average concentration of nitrogen oxides in the flue gas can be reduced from 101.6 mg/m3 to 53.3 mg/m3. When the combustion air moisture content is 40 g/kg, the gas boiler operation is relatively stable, and the average concentration of nitrogen oxides in the flue gas is 70.1 mg/m3.