Effects of burning biomass (rice straw) in the field on content and composition of soil organic matter were studied to explore changes in content and structure of humic acid (HA), fulvic acid (FA) and dissolved organic matter (DOM) in relation to frequency of burning. It was found that change in soil organic carbon occurred only within a few millimeters of surface soil layer. It decreased in content and the magnitude of the drop increased with frequency of burning, while soluble organic matter and its components increased in content. Humic substances from different sources responded differently to burning. The humic substances in three studied soils didn’t show any consistent variation trend with frequency burning, but HA and FA contents, HA/FA and E4/E6 increased or decreased regularly with increasing frequency of burning. The change in FA content was the leading factor affecting HA/FA ratio. Besides, from the FTIR, it was learnt that in FA aromatic fractions and Aliphatic substances, like C—OH, C—O, increased as frequency of burning increasing, while changes in HA were not noticeable because the aromatic condensation of HA per se is high.