Abstract: The effects of applying potassium fertilizer at peak bolling stage(28,July) on cotton photosynthetic characteristics and its yield and quality under normal density(3.75104 plants/ha) removing monopodium and low density(2.25104 plants/ha) retaining monopodium were studied under filed condition with random block design.The K application rate was 75 kg/ha.The results showed that applying potassium fertilizer at middle growing stage retarded leaves aging,remained higher LAI at later growing stage,significantly increased SPAD value,photosynthetic rate,PSⅡ activity((FV/FO),actual photochemical efficiency of PSⅡ(ФPSⅡ),stomata conductance(Cs) and significantly decreased non-photochemical quenching coefficient(NPQ) in stem leaves at later growing stage for both cotton populations.As the result,light energy utilization efficiency of cotton population and leaves at later growing stage were enhanced,total bolls and boll weight were increased remarkably,cotton lint yield were increased by 8.9%～9.3%,and cotton fibers tenacity was improved.