Abstract: As a vital biological resource in agricultural production system, crop straws returned into soil are crucial for maintaining and improving soil fertility and crop yield. There are differences in straw decomposition between anaerobic and aerobic conditions. FTIR (Fourier transform infrared spectroscopy) can indicate the dynamic changes in organic components during decomposing process of straws. In this study, the net-bag method was used to determine the decomposition dynamic of rapeseed straw and the characteristics of FTIR under anaerobic and aerobic conditions. The results showed a rapid decomposition rate of rapeseed straw at the start of the decomposing process, and then a slow rate with time. Within 360 days of decomposition, the rates of decomposition of rapeseed straw under anaerobic and aerobic conditions were respectively 60.50% and 68.20%, with corresponding decomposition rate constants (k) of 0.004d^-1 and 0.010d^-1, and rapeseed straw mass decomposition half-lives (t 1/2) of 229 days and 117 days. Carbon (C) releasing rates of rapeseed straw under anaerobic and aerobic conditions were 70.33% and 77.43%, respectively. Also C releasing rate constant (k) under anaerobic condition (0.025d^-1) was smaller than that under aerobic condition (0.026d^-1). Then nitrogen (N) releasing rates of rapeseed straw under anaerobic and aerobic conditions were 82.20% and 87.48%, respectively. Also N residues under anaerobic condition was 38.25% higher than that under aerobic condition (P < 0.05) within 360 days decomposition, with the highest difference during the 6090 day incubation period. Infrared spectrum analysis showed that absorption intensities at 3 4303 410 cm1 (stretched hydroxyl), 2 930 cm^-1 (stretched methylene) bands significantly decreased. This suggested that the decomposition of organic matter such as carbohydrate and aliphatic compounds decreased. Absorption intensities at 1 4191 425 cm^-1 and 1 740 cm^-1 (assigned to C=O stretching as well as C—O stretching and C—H deformation of carboxylic acid functional groups) weakened. This suggested that carboxylic acid lipid compounds and lignin contents decreased and that there was higher absorption intensity under anaerobic condition than aerobic condition. Silicon released from rapeseed straw accumulated as oxides such as SiO₂. During the process of straw decomposition, aliphatic compounds declined while aromatic compounds increased. Also aerobic condition was beneficial to the decomposition of straw cellulose and hemicellulose, largely supplementing soil C and N.