Although the increasing trend of sensor signal with decreasing oxygen partial pressure was observed quite early, the underlying mechanism is still elusive, which is a hindrance to accurate gas detection under varying oxygen partial pressure. In this work, a sensing model based on previous experimental and theoretical results is proposed, in which the O2-species is determined to be the main oxygen species because O-species has not been observed by direct spectroscopic studies. On this basis,combined with the band bending of SnO2at different oxygenpartial pressures, the functional relationship between the surfaceelectron concentration, oxygen partial pressure, and reducing gasconcentration is established, which includes three forms corre-sponding to the depletion layer, accumulation layer, andflat band.In the depletion layer case, the variation of the sensor resistance to different concentrations of CO and oxygen can be wellfitted withour function model. Besides, this model predicts that the response of sensors will no longer maintain the increasing trend in anextremely hypoxic atmosphere but will decrease and approach 1 with the background oxygen content further going down to 0