Abstract: The pyrolysis processes of cotton and flax fabrics, which are typical kinds of biomass solid waste, were studied under low pressure by tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The thermogravimetric analysis (TGA) results show that flax fabric has larger proportions of hemicellulose and lignin components compared with cotton fabric. Main pyrolysis products were identified with mass spectra obtained at different photoionization energy together with GC/MS. Temperature-dependent mass spectra of the products show that the most insensitive pyrolysis products of cotton fabric are observed at 500 ℃. Under the effect of hemicellulose and lignin components, flax fabric has a relatively wider temperature range. Moreover, hydroxyacetaldehyde (HAA) always exhibited very minor quantity，which confirms that HAA is a secondary pyrolysis product of cellulose.