The significance of fuel-air ratio on the study of internal combustion engine was discussed firstly and the measuring methods of fuel-air ratio were classed according to the measuring objects or measuring locations. The effects of the vapor content in the air and the atomicity ratio of H, O, N to C of the fuel on stoichiometric air-fuel ratio were investigated. In terms of elementary theory of combustion chemistry, the equation for calculating the excess air factor of the fuel-air mixture before combustion from the oxygen content in the heated and oxidized combustion products was deduced. Besides, the deduced equation was used to determine the effects of water vapor content in the air, NO emission content of the combustion products and the atomicity ratio of H, O, N to C of the fuel, etc. on excess air factor. At the end of this paper, another equation for calculating the excess air factor using the volume fractions of CO2, CO, O2, NO, NO2, HC measured from the combustion products and water vapor in the air was deduced. And the observed values of CO2, CO, O2, NO, HC in the engine exhaust were also put into the latter equation to calculate the excess air factor. At the same time, a comparison between the predicted from the equation and the observed from the exhaust gas analyzer was made.