The ascorbic acid molecular structure had been studied by infrared spectrum (IR spectrum). The infrared absorption modes of ascorbic acid were found, which are  νC=O-ascorbic acid、 νC=C-ascorbic acid、 νasC-O-C-ascorbic acid、 βmono-OH-ascorbic acid、νsC-O-C-ascorbic acid and νring-ascorbic acid. Temperature-dependent infrared spectrum (TD-IR spectrum) of ascorbic acid were studied also. With the increase of the temperature (303 ~ 393  K), the infrared absorption intensity and frequency of ascorbic acid did not changed obviously. The key roles of IR and TD-IR spectrum in the analysis of structure and thermal stability of the important organic compound (ascorbic acid) were demonstrated.