A detection method and detection device for the reaction concentration of gas, wherein the detection method comprises the following steps: step 1) detecting in real time total volume flow rates Q1 and Q2 of gas reactants before and after a reaction; step 2) calculating a real time volume flow rate difference ΔQ of gas before and after a chemical reaction; step 3) determining the relationship between a volume change value ΔV of the gas before and after the reaction and the volume V of gas to be detected, and calculating a volume flow rate Q3 of the gas to be detected; step 4) equating each gas reactant to an equivalent reactant according to a ratio of stoichiometric numbers corresponding thereto; and step 5) calculating a gas mass percentage of the gas to be detected. By means of the change in the volume of gas before and after a chemical reaction, the described method solves the problem in existing technology in which detection may only be performed by means of sampling, thereby improving the accuracy and stability of detection.