The suitable middle leaves (No. 9–11) of Yunyan85 were used as experimental materials, and the tobacco leaves located on the top layer of the four–layer air rising type curing–barn were observed for mildew every 4 h by camera, when the number of mildew leaves in two successive observations was not amplified (60 h), the observation was stopped. The ultrastructure of the midrib and epidermis of the tobacco leaves were observed every 12 h by scanning electron microscopy, at 60 h, the test was terminated. The results showed that during the curing process, tobacco leaves began to mildew at about 22.3 h, the mildew outbroke at about 43.47 h, the mildew rate was 29.2%, and tobacco leaves stopped mildew around 56.7 h with mildew rate 58.2%. After calculation, the mildew rate curve conforms to the “S” type curve; at 0–12 h, the midrib of the tobacco leaves were damaged and some crystals were produced, in 12–24 h, crystals were cleaved and hyphae appeared, at 48–60 h, rapid growth and reproduction of mildew was happened and the amount of epidermal mildew was the largest; in 0–12 h, the width of the epidermal cells in the midrib of the tobacco leaves contracted slowly with a slight amount of mildew spore germination, rapid contraction occurred after 12 h, hyphae appeared in 12–24 h. mildew rapid growth and reproduction period occurred between 48–60 h, at 60 h, the glandular hairs on the epidermis were shrivelled. The occurrence time of tobacco mildew is in the middle and late yellowing period, the best time to control tobacco mildew is before 22.3 h.