Abstract: Knowledge of K requirement for cotton growth and development is need since it play a pivotal role on its efficient production and fiber quality．Our present study focused on the effects of potassium nutrition on cotton growth and leaf physiological characteristics at seedling stage．Two cotton varieties，Lul8(L18)and zhong41(ZH41)，were used and four rates of K fertilizer were applied(K2O O，60，180 and 300 kg/ha)under field condition with randomized block design．The results showed，K application increased plant height and leaf numbers per plant，but its effects on leaf area were not consistent．It appeared to reach maximum area when K application rate was 180 kg/ha，and then declined with further increasing potassium fertilizer．K application could promote the synthesize of chlorophyll．K application increased the chlorophyll fluorescence dynamics parameters，Fv/Fo，Fv/Fm and qP，and reduced qN．K application also increased actual quantum efficiency(PSⅡ)and photosynthetic electron transport rate(ETR)，which improved the photosynthesis of cotton leaf consequently．In addition，the effects of K application on endogenous hormones were divergent．It increased the content of IAA，ZR，and GA3，while the content of ABA were reduced as the result；The lower ratio of IAA/ABA，ZR/ABA，GA3/ABA ratio indicated that senescence of leaf could be retarded．Rational application of K could promote N metabolism through increasing the activity of NR．