Abstract: Field trial, sand culture and plant analysis were used to study the effect of N and K interaction on leaf senescence and its mechanism emphasized on aspects of endogenous hormones and metabolism of active oxygen in spring maize. The results indicated that rational ratio of N to K could increase contents of chlorophyll, IAA, ZR and GA3, and lower content of ABA. With rational ratio of N to K, the ratio of IAA/ABA, ZR/ABA and GA3/ABA increased, the activity of POD and the content of MDA which is the product of lipid peroxidation decreased, while the activity of SOD enhanced, which delayed leaf senescence. However, the balance of endogenous hormones was broken, and the ability to scavenge free radicals declined, resulting in early senescence and lowered grain yield with irrational ratio of N to K.