By on-line infrared spectroscopic method，the changes of chemical constitution of acy-lonitrile (AN)／diacetone acylamide (DAA)／methyl acylate (MA) copolymers have been measured continuously during its lOW temperature pyrolysis under different atmospheres．From the results it was found that during the pyrolysis dehydrogenation and oxidation re-actions proceeded mainly after cyclization reaction under atmospheric environment，but pro-ceeded before cydization in pure oxygen．In nitrogen atmosphere，only cyclization and dehy-drogenation occurred．It was shown that the length of cycloforming sequence of copolym-ers pyrolyzed in nitrogen before 275℃ was slightly shorter，and after 275℃ was slightly longer than that in other atmosphere．Based on the variation of intensities of characteristic peaks with heating time，it is obvious that the increment of relative content of carbonyl group in the samples is proportional to the square root of pyrolytic time．