Abstract: Recently,potassium deficiency had become an important problem limiting cotton production in Huanghuaihai Plain,China.In order to elucidate the effects of potassium applicaiton on the photosynthetic characteristics,yield and fibre properties of different genotypeic cotton,field experiments were carried out on a sandy loam soil at Henan Agricultural University Experimental Station during 2004-2005.The content of exchangeable potassium in the native soil was 101.5 mg/kg.Three cotton genotypes,CCRI 38(a transgenic insect-resistant hybrid variety),DP33B(transgenic insect-resistant variety) and CCRI 12(traditional variety) were used in experiments.The photosynthetic functions in the stem leaves of cotton were investigated by means of photosynthesis and chlorophyll fluorescence determination method at desired dates.The results showed that basic applying KCl 225 kg/hm2 before cotton seedlings transplanted increased LAI,PSⅡ maximal photochemical efficiency(Fv/Fm),PSⅡ potential activity(Fv/Fo),PSⅡ actual photochemical efficiency(ФPSⅡ),net photosynthetic rate(Pn),stomatal conductance(Gs) in leaves,accumulation of biomass and the proportion of distributed to reproductive organs in plant,and total bolls per hectare,boll weight and lint percentage.Lint yield of CCRI38,DP33B and CCRI12 were increased by 13.60%,10.66% and 4.21% respectively.In addition,fiber qualitative characters,such as 2.5% span length,tenacity,micronaire value and length uniformity were also improved.CCRI38 and DP33B responded more active to potassium fertilizer than CCRI12.