A wall-hung gas boiler was innovatively proposed and designed in this paper. Water-cooling premixed combustion and enhanced condensation heat exchange technology were adopted in the boiler. The extruded aluminum plate-fin structure was adopted in the burner and condensing heat exchanger. Experiment and numerical simulation studies were conducted on the flow, combustion, and heat exchange characteristics of the boiler. The effect of the fin structure, excess air coefficient, heat load, and water-cooling temperature on the thermal performance of the boiler was analyzed. The results showed that reliable ignition, stable combustion, uniform flame distribution, and low pollutant emissions can be achieved in the wall-hung gas boiler. Affected by burning intensity and internal flue gas re-circulation, when the burner plate-fin gap was 1.63 mm, the flame was the shortest and the NOx emissions were the lowest. Under this fin condition, ultra-high efficiency and ultra-low emissions can be achieved in the boiler with a low excess air coefficient. When the excess air coefficient was 1.3, the NOx emissions were less than 30 mg/m3 at the heat load of 4-14 kW, and the thermal efficiency can reach up to 102.8% at the rated load of 14 kW. Within the scope of the experiment, the NOx emissions changed little with the water-cooling temperature. At the temperature of 333 K, there was still a great NOx emission reduction effect.