The invention discloses a method for online measurement of concentration of OH free radicals in a flame zone of Class B fire and a flame device. The method adopts the technology of laser-induced fluorescence to make a laser beam with the absorption wavelength of 308nm tuned to OH irradiate OH free radicals in a flame zone, so as to generate resonant transition from a low electronic state to a high electronic state. The excited molecules then self-radiate fluorescent. Under the condition that laser intensity and instrument conditions are maintained unchanged, the total fluorescence intensity is proportional to the concentration of the OH free radicals, thereby obtaining the concentration of the OH free radicals. The device of the invention comprises a pear-shaped funnel (1), a square combustion device (3), an oil cup (5), a mist rising control tube (6), a bracket (7) and a humidifier (8). The invention has the advantages of high measurement accuracy, simple structure and easy implementation.