The invention discloses a method for detecting free radical ration in 
coal tar. The method comprises the steps of removing 
moisture in the 
tar in a high speed centrifuging manner, and preserving a 
tar sample under low temperature or 
inert atmosphere; adopting a 
diamagnetism substance to deliquate 
DPPH(1,1-Diphenyl-2-picrylhydrazyl radical 2,2-Diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl) to prepare a plurality of 
DPPH standard samples with different free radical concentrations, carrying out EPR (
electron paramagnetic 
resonance) measurement, carrying out secondary integration on an EPR 
spectrogram, and recording the corresponding secondary integration area; establishing a 
standard curve according to the free radical contents of the 
DPPH standard sample with different free radical concentrations and the secondary integration area of the corresponding EPR 
spectrogram; and filling the tar sample into a 
pipe, measuring according to the same EPR measuring condition, carrying out secondary integration on the measured EPR 
spectrogram, and substituting the secondary integration area into the 
standard curve so as to obtain the free radical content N. The method provided by the invention can exactly measure free radical content in 
coal tar, meanwhile, the change of the content of the free radical in the 
coal tar along with the preservation condition is exactly measured, and the method for detecting ration is provided for the research of the free radical in the 
coal tar and is very simple and easy.