Synchrotron radiation soft X-ray and SR-FTIR spectroscopy technique were used to study the change of soil organic carbon(SOC) structure in ＜0.25 mm aggregate soil from fallowing grasslands of Yun-wu mountains. Results obtained by SR-FTIR technique were qualitatively analyzed, which indicated that compared with slope farmland, the peak intensity of aliphatic-C from fallowing grasslands significantly increased, while peaks intensity of aromatic-C, alcohols-C, carboxyl-C, polysaccharide-C didn’t significantly change with increasing years of fallowing. Results obtained by SR-FTIR technique were semiquantitatively analyzed, which showed that compared with slope farmland, the relative content of functional groups from fallowing grasslands SOC was increased, the growing rate of aliphatic-C, alcohols-C in 0～5 cm and 10～20 cm soil horizon was greater than growing rate of aromatic-C. Results obtained by soft X-ray technique are consistent with SR-FTIR, and the relative content of the same functional groups (aliphatic-C, aromatic-C) gained by two determination methods were T-tested, which was not significant (P>0.05), indicating that using synchrotron radiation soft X-ray for the determination of SOC structural change of fallowing grasslands is feasible.