Abstract: In order to resolve the problem of over-darkening during tobacco curing, polyphenol oxidase inhibitors, l-cysteine and thiourea, were applied on tobacco leaves before flue-curing, and the polyphenol content, appearance quality and smoking quality of cured tobacco leaves and polyphenol oxidase activity during curing were analyzed.The results showed that the contents of total polyphenol and total chlorogenic acid increased obviously, the appearance quality and smoking quality improved slightly, and the activity of polyphenol oxidase decreased somewhat, with spraying l-cysteine onto tobacco leaves at 4.0 and 8.0 mmol/L before flue-curing.