The invention provides a method for obtaining a mapping relation between a spectrum and a concentration. The method for obtaining the mapping relation between the spectrum and the concentration comprises the following steps: (A1) gas is introduced into an acid solution, and water and acid mixed gas escapes from the liquid surface and is transmitted to a gas chamber through a heat tracing pipeline;(A2) a spectrum of the water and acid mixed gas in the gas chamber is obtained; the water and acid mixed gas is collected and is analyzed to obtain a concentration value of acid gas; and (A3) a waveband is selected in the spectrum, and in the wave band, acid is not absorbed and water is absorbed; and a spectrum of pure acid is obtained by analyzing the spectrum and a spectrum of the water, so that the mapping relation between the spectrum of the pure acid and the concentration is established. The method has the advantages of being accurate, convenient and the like.