Surface acoustic wave (SAW) ammonia gas sensors based on ZnO/SiO2 bi-layer nanofilms on ST-cut quartz surface acoustic wave devices were fabricated and characterized. The ZnO and SiO2 layers were coated onto SAW resonators by combining a sol–gel process and a spin-coating technique. The SEM and AFM results revealed the ZnO/SiO2 bi-layer films had porous structures. The gas sensing results showed that the sensitivity of sensors was dependent on the value of sheet conductivity of the sensing films. As a result, the bi-layer nanofilms were much more sensitive than the single layer films due to their appropriate sheet conductivity, and the absolute response value was dependent on the thickness of the top ZnO layer. The sensor based on the bi-layer nanofilm with 60 nm top ZnO layer showed the best gas sensing property. It exhibited a frequency shift of 2000 Hz in 30 ppm ammonia gas with good repeatability and stability.