【Objective】So far little has been reported on accumulation and transformation processes of soil organic matter in ancient paddy soil which has been buried or covered under paddy conditions as a result of environmental evolution, geological alteration or natural disasters. The organic matter in ancient paddy soils that have long been under obturation has already altered in nature and composition. Infrared spectrum is an important means that can be used to characterize chemical structure and physical properties of polymers. As organic matter in the soil absorbs the different wavelength spectra selectively, a large amount of information about the nature, reaction characteristics and structure of oxygen-containing functional groups can be extracted from infrared spectrograms for identification. Since the organic matter in the buried ancient paddy soil is very complex in structure, how to analyze various functional groups in organic matter is a significant problem that needs to be solved in the present study. 【Method】The soil organic matter in the ancient paddy soil profiles at Sanlonggang of the Liyang Plain, demonstrated clean traits of accumulation and transformation as affected by cultivation age, environmental condition, and burial depth. So soil samples, were collected from the ancient paddy soil layers and the modern paddy soil layer of the profiles for infrared spectral analysis using the potassium bromide pressed disc method, to identify functional groups in the organic matter therein and characterize structures of the groups separately, in an attempt to provide some theoretical and practical bases for revealing structural characteristics and evolution processes of the organic matter in the buried ancient paddy soil. 【Result】Results show that the organic matter in the ancient paddy soil has become an inherent component of the soil under long-term burial and carbonization and hard to get leached, thus leading to accumulation of the matter relative to the modern paddy soil. The vibrations in the infrared spectrum of the ancient paddy soil are mainly composed of those of halogen and inorganic compounds, and some others of functional groups like olefins, aromatics and hydroxyl compounds. The wave at Band 1 882 cm^-1 (carbon olefin) of the infrared spectrum of the ancient paddy soil weakens obviously after organic matter is removed from the soil and the vibration of functional groups is mainly the stretching vibration of unsaturated C-H. In the infrared spectrum of the original red soil, the absorbance is lower with the ancient paddy soil than with the modern paddy soil at the wave band 3 620～1 631 cm^-1, but higher with the former than with the latter at the wave band of 1 631～470 cm^-1, which suggests that the benzene rings are gradually forming with alkenes gradually aromatized and halogen and inorganic ions bound more intensely. First-ordered reciprocal (FD value) of the infrared spectrum shows that the ancient paddy soil does not vary much from the modern paddy soil; After organic matter is removed from the ancient paddy soil, peaks in its infrared spectrum are all weakened, especially FD peaks, which all drop below their respective ones in the infrared spectrum of the soil without removing organic matter, which demonstrates that the functional groups bound with organic matter are weakened in the soil after organic matter is removed, and the presence of organic matter can highlight the infrared spectral features of the structural functional groups of organic matter. 【Conclusion】In the infrared spectrum of the soil with organic matter removed, the ancient paddy soil is lower than the modern paddy soil in absorbance at all wave bands, which suggesting that organic matter has a greater influence on functional groups in the ancient paddy soil.