Effect of environmental humidity and water content in tobacco leaf on starch metabolism was studied by using the electric-heated auto-controlled flue-curing barn supplied by Henan Agricultural University, China. The results indicated that the degradation of starch and decrease of water content in tobacco leaf during early yellowing at low humidity was the most rapid, and the water loss was the highest while the lowest under high humidity. The duration for starch degradation under low humidity was longer than that of CK. So the starch residue in cured leaf of different treatment took the order of high humidity yellowing treatment＞low humidity yellowing treatment＞ CK. When the leaf water content was decreased to around 50%, the starch degradation became slow and the content of starch was stable. Starch degradation and decrease of leaf water content was not synchronous. Starch in tobacco leaf during yellowing degraded more rapidly when humidity was decreased at a high speed, but the degradation stopped earlier at late stage. There was a quicker and higher degradation of starch under high environmental humidity. When the humidity decreased to 70 %,the content of starch was stable. The activity of amylase began to decrease when relative humidity was below 75 %, but it kept a high level of activity when the environmental humidity was below 70 %.