The present paper reports a rapid method for the determination of furfural (F) and 5-hydroxymethyl furfural (HMF) in pre-extraction liquors of lignocellulosic biomass based on UV spectroscopy. In a concentrated acetic acid medium,F and HMF have an isosbestic point at 276 nm. It was found that the acidic soluble lignin in the pre-extraction sample is the major interference species in the F and HMF spectroscopic quantification. However,only acidic soluble lignins have the absorption at the wavelengths above 325 nm. Based on the absorption of the acidic soluble lignins at 325 nm,their absorptions at either F or HMF absorbed wavelengths can be determined. Thus,with a simple triple-wavelength technique,both F and HMF in the pre-extraction liquors of lignocellulosic biomass can be quantified based on the spectroscopic measurement at the isosbestic point wavelength (276 nm),maximum absorption wavelength of F (272 nm) and the acid soluble lignin absorbed wavelength (325 nm). The present method does not require the hazardous organic compounds (such as phenolic compounds etc.) acting as a color reagent in the experiment. It is not only simple and rapid, but also has a good measurement precision and accuracy,with the relative standard deviations of 3.02% and 2.72%,and recoveries of 95%-107% and 96%-101%,respectively,in the F and HMF quantification. The present method is suitable for use in the research on pre-extraction hemicellulose of the lignocellulosic biomass in bio-refinery area in order to achieve a high selective sugar conversion.