Abstract: N and K play key roles in the quality and senescence of tobacco leaves. Pot culture and field experiments were conducted in Xundian County and Kunming City of Yunnan Province to study the ripe mechanism of tobacco leaves regulated by N and K nutrition from the changes of endogenous hormones and chlorophyll content in the leaves. Results showed that the content of N, K, chlorophyll, endogenous CTK decreased in the process of leave senescence, while the content of endogenous ABA took on a liner upward tendency. Multiplying application of N and K fertilizers could effectively improve N and K trophic level, increase chlorophyll and CTK content and reduce ABA content of tobacco leaves, thus retard the senescence of tobacco leaves. There was a significant positive correlation between N, K content and chlorophyll, CTK content and CTK/ABA value, however, the correlation between N, K content and ABA content in tobacco leaves was significant negative. Thus, the content of endogenous hormones and its balance in tobacco leaf were close related to N and K nutrition. Altering the dosage of N and K fertilizers could regulate senescence rate of tobacco. It was also shown that no matter single increasing the dosage of K fertilizer or increasing the K dosage coordinated with N and P fertilizers pro rata, K content of leaves increased remarkably, especially in the prophase. The transport potential of K of old leaves was very high even under the condition of high K fertilizer input. It could be inferred that leave-spraying CTK or ABA should be a feasible method to change the senescence rate and improve the quality of tobacco leaves when the tobacco plant couldn't ripe at due time because of the improper application of nitrogen or potassium fertilizer.