Abstract: 【Objective】 Simulated experiments were conducted to clarify the structural characteristics of organic sub‐ stances of three crop straws during the decomposition process so as to provide theoretical basis for rational utilization of tobacco field straws. 【Method】 Using tobacco，rape，and rice straws as research objects，the nylon mesh bag method was used to simulate decomposition for 180 d. Fourier transform infrared spectroscopy and thermogravimetric（TG）analysis were applied to study the differences in decomposition characteristics of the three crop straws under two soil moisture conditions of dry wet alternation and flooding. 【Result】 The decomposition rate of three straws at 180 d reached 55.69% to 69.39%，with rice straw>rape straw>tobacco straw. The decomposition rate of the same straw under two different soil moisture conditions showed wet dry alternation treatment>flooding treatment，but the difference was small. There were similarities in the functional group composition of the three crop straws，sharing absorption peaks at 3420，2920，1640， 1380，1050，and 610 cm^-1 revealed by infrared spectroscopy analysis. However，the relative intensities of some characteristic absorption peaks and absorption peaks were varied，showing that the corresponding material and content of these absorption peaks were different. The ratio of relative intensities of 1640 cm^-1/2920 cm^-1 and 1640 cm^-1/1050 cm^-1 in tobacco straw were the highest，indicating a higher content of aromatic compounds of tobacco straw. As the decomposition progressed，the infrared spectrum characteristic absorption peaks and their relative intensities of the three straws changed. Carbohydrates，amide compounds and aliphatic compounds gradually decomposed to form carbonates，ammonium salts， nitrates and carboxylates. The element silicon in rice straw was released to form silicon dioxide and silicates. The TGDTG curve analysis showed that the weight loss during the 200-400 ℃ stage of the three straws reached over 55.00%， which was the main stage for the release of volatile matter from straw. The maximum weight loss during the stages of 200- 400 ℃ and 600-700 ℃ were found in rape straw（58.71%）and tobacco straw（6.38%）respectively，while the residual mass percentage at 900 ℃ of rice straw was the highest（23.17%）. At the late decomposition stage（180 d），there were significant differences in the content percentage of fixed carbon and volatile matter of the three straws（P<0.05），while there was no significant difference in the same crop straw under two different soil moisture conditions（P>0.05）. Among them，rice straw had the highest fixed carbon percentage，reaching 17.32%，with the lowest volatile matter percentage. The volatile matter percentage of rape straw was relatively high，with the lowest fixed carbon percentage average of 11.69%. 【Conclusion】 Infrared spectroscopy combined with thermogravimetry can be used to analyze the degradation status of simulated returning straw to the field. Tobacco straw is more difficult to decompose，while returning rice straw to the field is more conducive to increasing soil organic carbon content.