Thermal-oxidative ageing of polyoxymethylene (POM) copolymer in oven at 100 °C for 1, 2, 3, 5, 7, 10, 14 and 21 days and the influence of early thermal-oxidative on POM structure and properties were studied by means of wide-angle X-ray diffraction (WAXD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and tensile test. Based on the results, we found that the early thermal-oxidative ageing of POM copolymer can be divided into three regions. The first region is the initial three days. In this region, some molecular chains rearranged, resulting in internal stress relaxation, increased of crystallinity degree and grain size due to the perfection of crystal structure; both extended chain crystal (ECC) and folded chain crystal (FCC) increased and ECC grew faster than FCC. The second region is from 3 days to 10 days and in this region, chain scission took place in amorphous region and led to chemi-crystallization. The last region is after 10 days. In this region, the structure and performance of POM copolymer reached a stable situation at this stage. In this work, the difference between skin and core were also analyzed.