Formaldehyde (HCHO) is widely used in construction, wood processing, furniture, textile, and carpeting industries. However, it is highly toxic. It strongly irritates human eyes and nose, and is a carcinogen. In this paper, the effects of gas concentration and operating temperature on the sensing properties of the nano-SnO2 flat-type coplanar gas sensor arrays to formaldehyde were studied. The results revealed that the nano-SnO2 flat-type coplanar gas sensor arrays exhibited good sensitivity such as a fast response, short recovery time, and low detection limit. In addition, the adsorption and surface reactions of formaldehyde on SnO2 films were also studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at 200–300 °C. Molecularly adsorbed formaldehyde, formate, dioxymethylene, polyoxymethylene, H2O, and CO2 surface species were formed during formaldehyde adsorption at 200–300 °C. Moreover, a possible mechanism of the reaction process was given.