NO_x emission reduction from gas boilers has become a key issue in improving air quality. Combustion air humidification technology is gradually being used to reduce NO_x emissions. However, the NO_x emission reduction effect of gas boilers at a higher combustion air humidity has been studied less. A flue gas with low NO_x emissions and a waste heat recovery system using combustion air humidification technology are proposed in this study. In the ultra-low NO_x mode, the effect of high combustion air humidity on NO_x emission reduction and efficiency of the gas boiler were studied experimentally. In the waste heat recovery mode, the effects of the heat network backwater temperature on the NO_x emission reduction and system efficiency were studied experimentally. Results showed that an increase in air humidity can significantly reduce the NO_x concentration formed by combustion. The ultra-low NO_x mode reduces NO_x emissions from 130 mg/m³ to 23.3 mg/m³ and affects the boiler efficiency slightly. In the waste heat recovery mode, NO_x emissions can be reduced to 39.9 mg/m³ when the backwater temperature of the heat network is 55 °C. This condition improves the efficiency to 93.8%. The analysis results provide suggestions for the selection of the operation modes.